Mediterranean stock assessment: current status [PDF]

MEDITERRANEAN STOCK ASSESSMENT: CURRENT STATUS, PROBLEMS AND PERSPECTIVE

Abella Alvaro J. Arneri Enrico Belcari Paola Camilleri Matthew Fiorentino Fabio Jukic -Peladic Stjepan Kallianiotis Argyris Lembo Giuseppe Papacostantinou Costas Piccinetti Corrado Relini Giulio Spedicato Maria Teresa

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ARPAT-GEA Livorno - Italy CNR - IRPEM Ancona - Italy Università di Pisa Pisa - Italy Centre for Fisheries Sciences Marsaxlokk - Malta CNR - IRMA Mazara del Vallo - Italy Institute of Oceanography and Fisheries Split - Croatia NAGREF - Fisheries Research Institute Kavala - Greece COISPA Tecnologia & Ricerca Bari - Italy National Centre for Marine Research Athens - Greece Laboratorio di Biologia Marina e Pesca Fano - Italy Università di Genova Genova - Italy COISPA Tecnologia & Ricerca Bari - Italy

Foreword

In the 26th session held on 10-13 September 2001 (Lacco Ameno, Ischia, Italy), the Scientific Advisory Committee (SAC) was requested by the General Fishery Commission for the Mediterranean (GFCM) “to review existing stock/fisheries assessment methods, highlighting data needs, outputs expected, pros and cons, and their suitability to Mediterranean stocks and fisheries. On the results of the above analysis, SAC was requested to define a set of most appropriate assessment methods which should results in the establishment of a harmonised assessment methodology, agreed by scientists of Member, and which is a prerequisite for a rational decision-making process.” Following this request Mr. Pere Oliver prepared, on behalf of COPEMED, a working document on methods for assessing Mediterranean fisheries (Annex 8 of the Sub-Committee on Stock Assessment report, 2002). This document, circulated and discussed within the SCSA meeting held in Barcelona on 6-10 May 2002, concluded that, given the available information in the area, the length Cohort Analysis (LCA) is, at present, the only current option available to start assessing and providing advice in a harmonised way and on a regular basis at the regional level. The discussion on this contribution, within the SCSA, highlighted that no consensus was reached, thus the attendants to the meeting were invited to produce working papers on these matters to be sent to the SAC. The present working document has therefore the aim of furnishing to the SACGFCM a further contribute to progress towards the harmonisation of methodologies on stock assessment, viewing at the Mediterranean fisheries under a different perspective. The Authors are aware that, given the short time available and the complexity of the matters, this contribution can represent only a first approach to the problem and that further insights will be needed.

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A short overview on the main characteristics of Mediterranean fisheries

The case of the Mediterranean, one of the oldest and most intensively exploited marine system, is considered a paradox as many of its resources have been declared fully fished or overfished for decades, although production continue to rise slowly, probably in response to eutrophication, except fo r the Black Sea where pollution, species introduction and overfishing have played a part (Caddy et al., 1995; Grainger & Garcia, 1996). Despite the upward long term trends, short terms declines and increases were observed, more or less in the same proportion, over the last 5-8 years, the former especially in the western and northern regions, the latter mainly in the southern and eastern Mediterranean (Farrugio & Papaconstantinou, 1998). Mediterranean fisheries target different resources, most of them are pelagic stocks. These stocks are generally more abundant than demersal ones and show more important fluctuations in biomass, frequently not very well explained. Small pelagics (mainly sardines and anchovies) are the target of important industrial fisheries, purse seines and pelagic trawls are the most common fishing techniques for this group of species. Stock units of commercial interest are in general located in more restricted areas and their distributions are linked to major rivers run-off (e.g. Ebro, Rho ne and Po) and upwelling areas. Their geographical distribution often exceeds the jurisdictionary waters of a single country and, hence, they are frequently exploited by more than one national fleet. Co-operative assessment and management conducted by the countries that share some of these stocks have been attempted in the last years. A large number of stocks, especially composed by benthic and demersal species distributed over the generally narrow continental shelves characterising the Mediterranean Sea are exploited by industrial and artisanal fleets. These stocks usually remain within national waters, except in some areas characterised by a wider continental shelf (Caddy, 1993). In the case of semi-sedentary or territorial demersal resources living on the shelf, the distribution of fishing effort from local ports is almost exclusively exerted on the grounds positioned just in front of them. Spawning and nursery areas are in many cases, and for many species, located all along the shelves. Thus, the fishing pressure will impact mainly “local stocks”, producing changes in abundance, demographic structure of the populations and reproductive potential. Under an operative point of view, for these resources, the genetically based definition of unit stock simply as “self- reproducing units” might be replaced by the more practical definition: a self-perpetuating group, with limited interchange of adults from biomasses or cohorts of the same species in adjacent areas, within which the biological characteristics and impact of fishing is uniform (Gulland, 1983). In the Mediterranean coastal area, the target of the bottom trawl fisheries is often a species complex that have a major contribution in the total catch. It is also common that a single commercial species be caught by different fishing techniques or strategies that concentrating their fishing pressure on individuals of different sizes. The fishing activity addressed to the catch of certain species produces undesired mortality to many less valuable species. Most of them, with reduced or no commercial value, are partially or totally discarded at sea.

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An important fraction of the target species of the multispecies demersal fisheries is small finfish, cephalopods and crustaceans with high commercial value in the local markets. This fact has conditioned the choice of the selection capability of the gears in use. The mesh size currently used in the bottom trawl gears (legal mesh size is 40 mm) defines a high probability of retention for small-sized individuals, even for those belonging to species that potentially can reach relatively big sizes. For many species, juveniles which do not reach the size at first maturity compose most of the catch. Caddy (1993) after categorising three main groups of demersal species (fish with large size at first maturity captured much before the first spawning, e.g. hake, angler fish, etc.; species such as red mullet or Norway lobster that require at least a stretched mesh size of 40 mm; small species such as shrimps, cephalopods, gobies, etc., which could be completely lost increasing mesh size from 40 mm) concluded that the increasing of the mesh size, even to 60-70 mm, could be not effective in protecting the spawners of the “large fish” making, conversely, unavailable the “small species”. Thus, the effort should be reduced, and this measure should be complemented with the protection of young fish from exploitation and the protection of spawning stocks from the effects of extensive fishing. Other points that are worth mentioning, concern the possible fluctuations of the productivity in some Mediterranean marine systems. The limited ability of semienclosed systems to absorb massive discharges of nutrients, the climate change globally established, all of these affecting the validity of “equilibrium” approaches to stock assessment in the Mediterranean region. Moreover it is even possible that the enrichment of oligotrophic systems may in a early stage increases the production of both demersal and pelagic food webs, but if unrestrained it could lead serious problems for demersal and pelagic ecosystems (Caddy and Griffiths, 1990). Thus research efforts accounting for the environmental component in the stock assessment are advisable, especially in the light of the precautionary approach.

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Analysis of the assessment methods used in the Mediterranean

An indicative and simplified categorisation of stock assessment models includes: • simple or global models (e.g. biomass dynamics models); • analytical models based on estimates of population parameters (e.g yield per recruit; e.g. Beverton & Holt, 1957; Thompson & Bell, 1934) that encompass the family of the length and age-structured models (Length Cohort Analysis – LCA; e.g. Jones 1983, and Virtual Population Analysis – VPA; e.g. Pope, 1972). Of course, different variants of such models have been implemented and applied, using data from commercial fishery or from experimental surveys, the latter limited to the analytical approach. The diagnoses of fully exploitation or overexploitation of Mediterranean resources seldom, however, derive from assessments based on approaches accounting for the specific features and complexity of the ecosystems. Most evaluations of Mediterranean demersal resources to date are mainly addressed to monospecific stocks and have been based either on swept area estimate of abundance, or on some applications of analytical and production models or, more recently, on length-based methods. Although some food chain studies have been carried

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out, multispecies modelling has not yet been developed. The difficulties in producing assessments have been mainly related to the scarcity of information on different aspects of the fisheries and, to some extent, on species natural life history. The multispecific- multigear characteristics of most fisheries, the extremely dispersed landing sites, the small fraction of the catch that generally passes through organised fish markets, make catch assessments in the area particularly difficult. Due to traditional practices of fish sorting and selling, species separation is incomplete and this makes very difficult the direct estimation of total amount of landings by species. In order to obtain for each species, an unbiased sample of the size structure of their catch, samples proceeding from all the fishing strategies that capture each species are necessary. Sampling is, in most of the countries, very expensive considering the high price of the samples to be purchased and the high number of species of commercial importance for each fishing strategy. Discarding at sea is a common practice (even of individuals of many target species) and hence catch (and fishing mortality by age) for many species can not be estimated if the size composition and an estimate of the amount of discards is not available. CPUE is frequent ly used as an index of abundance. In the Mediterranean, however, fisheries target different species mixes, depending on the structural characteristics of vessels, gears in use, operational aspects, traditions of different groups of fishermen and market or other economical constraints. In such conditions, in order to estimate catch rates, an accurate partitioning of effort is necessary and the definition of the real effort directed to each single assemblage is not possible without this very detailed information. Another important problem is the general lacking in the Mediterranean of a systematic collection of data series on the main aspects of the fishery. Regarding the commercial data, current systems provide poor coverage of vital fleet, catch and effort statistics (Farrugio et al., 1993). Where available, catch trends were used for an analysis of the current status of the fisheries (Lleonart, 1999) or for short term catch forecasting (Stergiou et al., 1997; Lloret et al, 2000). In the North-Western Mediterranean, the Length Cohort Analysis, a simplification of the Virtual Population Analysis assuming the equilibrium condition, is currently widely applied. It requires a knowledge of catch distribution by size classes (pseudocohorts approach) and by gear and estimates of the biological parameters of the species (e.g. Jones, 1983; Lleonart, 1993). The development of such an approach in the North-Western side of the Mediterranean (Lleonart, 1993; Farrugio et al, 1994) was due to the EU research project FARWEST “Study for assessment and management of fisheries in the western Mediterranean” (1990-1994) that involved France, Spain and Italy. Thus also a package based on LCA and Y/R analysis adapted to Mediterranean fisheries, called VIT, was produced (Lleonart and Salat, 1992) and successively updated (Lleonart and Salat, 1997) up to the last version with the COPEMED support (Franquesa and Lleonart eds., 2001). The sampling effort of the 1980s has also enabled a few conventional VPAs to be applied to some stocks (Oliver, 1993; Aldebert and Recasens, 1994). In this analysis the results obtained when using VPA or LCA on annual pseudocohorts were compared, and in general, good agreement has been found between them (Oliver, 1994, Oliver et al,

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1995). Results from Length Cohort Analysis, however, must be handled with caution considering that most of the times only few years of data are available. It is likely that the use of “pseudocohorts”, the choice of biological parameters, the assumption of constant natural mortality M for all the ages, and constant recruitment, introduced in most of the analyses, have drastically conditioned the results. Many studies have shown discrepancies among the estimates obtained by VPA and those derived from direct methods as trawl and echo-surveys (Ulltang, 1996; Pennington & Strømme, 1997). VPA and similar techniques often provide biased estimates of the number at sea of younger ages due to undetected increases in mortality along the time, erroneous data of catches at age and of F or M estimates. VPA often exaggerates variability in recruitment reducing the probability of correctly detecting environmental correlates with recruitment. Lapointe & Peterman (1991) identified spurious correlations between fish recruitment and environmental factors due to errors in the natural mortality rate used in virtual population analysis. Mertz & Myers, (1997) suggest that the bias engendered by misspecification of M will vary temporally, introducing deceptive trends into the reconstructed series of recruitment. For the assessment of stocks in the Mediterranean, the utilisation of surplus production models became very popular in the seventies and eighties (e.g. Gharbi, 1985; Charbonier and Caddy, 1986). This choice was probably done with the aim to avoid some of the above-described problems related to the lacking of biological data at that time and to difficulties in getting age structure of the catches. These approaches generally did not furnish satisfactory results. The main problems can be related to unreal estimates of total catches, to the use of unsuitable effort units, to the unfeasibility of total effort partitioning, to the lack of enough contrasting data regarding effort and correspondent abundance levels, to the assumption of equilibrium, etc. Composite models (Munro, 1980) that use spatial information proceeding from ecologically similar sub-areas exploited at different rates can be used even in the case long data series on catch and effort are not available. For this reason and considering the above mentioned problems, they were used in the Mediterranean, by the first time by Caddy and Garcia (1982) and Garcia (1983), who assessed the state of exploitation, on a single species basis, in sub-regions of the Spanish coasts. Considering the difficulties of sampling commercial catches above described, programs based on the collection of fishery independent data (trawl-surveys, echosurveys, eggs and larvae surveys) were promoted at national and European level for estimating biomass and obtaining biological data. These researches carried out in the last decade on the Mediterranean demersal resources have significantly increased the knowledge on the distribution, abundance and population structure of several marine species, mainly those economically important. Most of the knowledge on such resources has been acquired by trawl surveys, in the context of both national (e.g. GRU.N.D. along the Italian coasts; Relini, 2000) and international projects (e.g. MEDITS; Bertrand et al., 2000). The MEDITS project started in 1993 aimed at the standardisation of the survey methodology among the different countries (France, Greece, Italy and Spain). Since 1996 and 1999 also data from the East Side of the Adriatic Sea (Slovenia, Croatia and Albania) and Morocco are respectively available. On 2000 the SAMED (Stock Assessment in the Mediterranean) EU project started. This program was based on MEDITS data and aimed at estimating

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demographic parameters of different target species in the Mediterranean, adopting common protocols. The output consists of a contribute to the assessment of the status of demersal resources. Results from the project were presented and discussed during the Seminar “Stock assessment and production of demersal resources in the Mediterranean” held at the FAO in Rome (March 2002) within the framework of the activity of the SubCommittee on Stock Assessment of the SAC. Generally, information from trawl-surveys is useful for mapping the spatial distribution of the species that may not coincide with the economic or administrative boundaries defined by fishermen or fisheries managers. In addition, trawl-survey provide indices of relative abundance over space and time and these estimates will reflect the real situation at sea, if the combination of sampling design and fishing procedures will allow to capture a portion of the stock proportional to the true abundance. Notwithstanding, relative indices can provide useful information to derive a picture of the variations occurring throughout the years, provided that sampling procedures and period do not change in time and species behaviour is considered. In the Mediterranean context, even if methods for using relative biomass index trends for stock assessment have not yet been well formalised, trends in abundance indices were used as indicators of the state of the fishery. Experimental trawl data are also suitable for different length or age based assessment methodologies, analogous to the length or age-structured stock analysis (ASA; Megrey, 1989) routinely applied when commercial catch data are available. The starting point is different, stock structure at sea for the former and catches composition for the latter, but basically the common goal is the estimation of the mortality of a given stock. The reconstructed size structure proceeding from trawl-surveys has been frequently used for the estimation of total mortality rates, deriving indications on the exploitation of resources. The accuracy of these estimates is however linked to the representativeness of the samples. Considering hake, for example, the problem of the different vulnerability or accessibility with increasing size should be accounted for, in order to avoid overestimation of the total mortality. However, even in the case of total mortality, detected variations throughout years can provide information for management, giving advice and allowing an adaptive strategy. Trawl-surveys with fine- meshed are the only source of information regarding the pre-recruited portion of the stock that can be combined with estimates of parental stock to detect stock/recruitment relationships. Moreover, information from trawl-surveys, associated with data on life history, allows the definition of boundaries and size of nursery areas or of any other aggregations due to sex, age and sexual maturity, useful for a correct management of the fishing activity (Hilborn & Walters, 1992). Nursery areas of different species encompassed or not inside the fishing grounds have been to date identified by different techniques accounting for the spatial dimension (e.g. Ardizzone and Corsi, 1997; Lembo et al., 1998; Lembo et al., 2000a; 2000b), and the importance of closure of these areas for juvenile protection has been highlighted. Time series of relative abundance of recruit (0 group) and adult (1+ group), gathered during the trawl surveys, allowed preliminary studies of stock-recruitment relationship for Mullus barbatus in some areas of Mediterranean, also taking into account environmental variability (Zamboni et al., 2000; Levi et al. 2001, submitted). Disadvantages of surveys can be represented by costs and variability associated with estimates that can also be high. Cook (1997) stated that trawl-surveys data are less prone to the biases affecting commercial catch data (mainly due to miss-reporting), but

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are subject to greater measurement error due to a smaller sample size. Another problem related to the use of trawl- surveys for the stock assessment and management is related to the lacking of ancillary information of fleets behaviour, demographic structure of the commercial catch, spatial and temporal distribution of fishing effort. On the other side, direct methods allowed the evaluation of exploitation impact on biodiversity in some areas, such as the Gulf of Lions and Adriatic Sea, comparing time series (e.g. Aldebert, 1997; Jukic-Peladic et al., 2000). Such information is very difficult to obtain by different approaches, as the key species used for the analysis are very often discarded. A better understanding of the ecosystem modifications induced by the fishing activity is a very important issue for the management, especially at the light of the precautionary approach. Information on the spatial distribution of abundance indices, if coupled with analysis on the geographical allocation of the fishing effort, could assume importance in stock assessment, allowing some variant application of the composite production models (e.g. Munro, 1980; Caddy & Garcia, 1982). Such an approach may help the evaluation on the status of resources exploitation along the Mediterranean coasts, where abundance data have been collected since many years (e.g. Relini, 2000; Bertrand et al., 2000) but no long data series of catch and effort data are available. An assessment of the state of the fisheries of Merluccius merluccius and Mullus barbatus that covered the whole western Italian coast and the eastern coasts of Corsica was recently performed by using trawl surveys data from MEDITS (Abella et al, 1999). The assessment was done using a Composite Model combined with the Caddy and Csirke (1983) variant of Surplus Production Model that uses the instantaneous total mortality rate Z as a direct index of effort, and catch per unit effort as an abundance index. This approach allowed calculating the situation of each single sub-area relative to the total mortality rate and, in particular, the Z position relative to the Maximum Biological Production (ZMBP ). As noted by Die and Caddy (1997) this reference point can be considered precautionary, as it corresponds to a slightly lower explo itation rate than the Maximum Sustainable Yield, and it is relatively stable and easy to calculate. Considering the approach by species, hake, red mullets and some Crustaceans (i.e. Norway lobster, blue and red shrimp, giant red shrimp, and deep-water red shrimp) are those more investigated among the demersals. For these species different kind of analyses, including assessments, have been carried out (e.g. Orsi-Relini and Arnaldi, 1986; Yahiaoui et al., 1986; Karlou and Vrantzas, 1989; Hadjistephanou, 1992; Martin and Sánchez, 1992; Oliver and Morillas, 1992; Recasens, 1992; Stergiou et al., 1992; Vassilopoulou and Papaconstantinou, 1992; Vrantzas et al., 1992; Aldebert et al., 1993; Demestre and Lleonart, 1993; Demestre and Martin, 1993; Sardà and Lleonart, 1993; Levi et al., 1995; Oliver et al., 1995; Papaconstantinou and Stergiou, 1995; Aldebert and Recasens, 1996a; 1996b; Ben Mariem and Gharbi, 1996; Ben Mariem et al., 1996; Abella and Serena, 1998; Bouaziz et al. 1998a; 1998b; Lleonart, 1999), many of them using data coming from the direct approach (e.g. Arneri and Jukic, 1986; Flamigni, 1984; Giovanardi et al., 1986; Matarrese et al., 1992; Ungaro et al., 1992; Levi et al. 1993; Tursi et al., 1994; Ungaro et al., 1994; Spedicato et al., 1995; Fiorentino et al. 1996; Ragonese and Bianchini, 1996; Tursi et al., 1996; Ungaro and Marano, 1996; Ardizzone and Corsi, 1997; Matarrese et al., 1997; Ardizzone, 1998; Colloca et al.,

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1998; D’Onghia et al., 1998; Fiorentino et al., 1998; Lembo et al. 1998; Orsi-Relini and Relini, 1998; Spedicato et al., 1998; Voliani et al., 1998; Abella et al., 1999; Zamboni et al., 1999; Ardizzone et al., 2000; Lembo et al., 2000a; 2000b; Papaconstantinou, 2000). Also for the small pelagics such as sardine and anchovy, different approaches have been used for stock assessment. Even in the case of these resources both the fishery-dependent data (Bouchereau et al., 1986; Chavance et al., 1986; Djabali et al., 1990; Pertierra and Perrotta, 1993; Cingolani et al., 1996; Cingolani et al., 1998; Sinovcic, 1998; Santojanni et al., 1999; Vidoris and Kallianiotis, 2000) and the direct estimates have been used. The latter approach has been applied considering the wide fluctuation of the biomass and the great variation of recruitment for biotic or abiotic reasons. Thus, the Daily Egg Production Method has been used to evaluate the Spawning Stock Biomass of fish stocks and biomass by hydro-acoustical surveys (Chavance, 1980, Oliver and Pastor 1986; Chavance and Girardin, 1986; Lazar et al., 1986; Miguel and Alvarez 1990; Regner, 1990, Miguel et al. 1991; Abad et al 1991; Abad et al 1992; Rubin et al 1992; Garcia 1992; Palomera and Pertierra 1993; Garcia and Palomera, 1996; Abad et al., 1996, Somarakis and Tsimenides, 1997; Casavola et al., 1998; Casavola, 1999; Quintanilla et al., 2000, Patti et al., 2000; Guennegan et al., 2000). A comparison between the DEPM, acoustic surveys and population dynamics models showed that their results appear to be quite consistent (Pertierra and Lleonart, 1996). An evaluation of the current knowledge on the shared stocks of small pelagics in the Adriatic Sea was recently addressed within ADRIAMED project (Mannini et al., 2001). In the Mediterranean, the catch composition of the trawl fishery is typically constituted by the first (0+, 1+) age classes, making the long term previsions (as those usually derived by using the classical approaches) sensitive to recruitment fluctuations and to the spatio-temporal variation of the fishing pattern. Moreover the estimation of demographic parameters such as natural mortality M should account for the above mentioned peculiarity. Considering the small size at first capture in the Mediterranean fisheries, it is likely that the heavily exploited juveniles of many species as hake or red mullet, suffer higher natural mortality rates than the older individuals due to both, density dependent processes and predation pressure. If natural mortality in younger ages is very high, a correct assessment of the consequences of alternative management actions must consider this highest natural mortality, as well as the functional relationship between preys biomass and predators density. As an example, if we assess the performance of changes in mesh size without considering the mentioned higher natural mortality rates of smaller individuals, the long-term gains associated with a mesh-size increase will undoubtedly be overestimated. Similar results have been proved if a reduction of fish mortality was applied (Fiorentino et al., 1996). In the analytical approaches more frequently applied, based both on retrospective analysis and conventional catch curve, this specific condition of the Mediterranean fisheries is often not taken into consideration. Some attempts to estimate natural mortality vectors for Mediterranean hake with alternative approaches (by using both, available biological information as fecundity and abundance estimates by size proceeding from commercial catch and trawl-surveys) were recently done (Abella et al. 1997, Caddy & Abella, 1999). There is, however, a need of an appropriate definition of the real state of the fishery related to the optimal fishing pattern and exploitation rate. Some authors have

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recently produced stock assessments based on yield per recruit and fecundity per recruit. The implications of the utilisation in yield-per-recruit computations of different hypothesis for the natural mortality rates at age (a constant value for the species whole life or a vector of the asymptotic or dome-shaped vulnerability-at-age) are very critical. Quite different results can be obtained regarding the definition of the current status of the fishery, as well as regarding the choice of an optimal combination of fishing mortality rate and mean size at first capture for hake (Abella et al, 1997) and for red mullet (Voliani et al, 1998). For hake, recent studies suggest that although fecundity per recruit would be increased somewhat by increasing the mesh size, it is not clear that this will necessarily increases yield per recruit markedly. Active measures would have to be taken annually to assure that fisheries targeted on larger fish do not deplete the spawning stock, in order to avoid recruitment overfishing.

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Data need, development of new approaches, and perspective for harmonising current assessment methodologies

Data requirements for fisheries assessment are more demanding and the collection of the basic information is generally costly. Despite the progress of the last decade, the available data in the Mediterranean are still far to be comparable with those of other areas such as the North Sea or United States coasts or Canada although, even in these situations, problems related to data reliability and accuracy are widely recognised (AA. VV., 1998). In these contexts, however, the joint utilisation of fishery-dependent and fishery- independent data is strongly recommended. Standardisation of approaches to data collection, monitoring, statistics compilation and research on fish population dynamics is needed in the light of the problems of overfishing and environmental degradation facing the Mediterranean. Catch per unit effort as well as length structure of catches are basic and essential information for stock assessment. The less expensive methods of obtaining commercial fisheries data are from reports of fishers about their landings (so called off-site method; AA. VV., 1998), whose limitations mainly consists of reliability and needs of further validation. Landing data can be also obtained from fish markets, where these structures exist and the product is effectively canalised by them. This source, however, needs to an independent quantification of the fishing effort and discards. The so called on-site methods enable the collection of more reliable and accurate data, but are much more expensive, requiring the sampling at the landing ports and on-board fishing vessels observers (Burns et al., 1983; Murawski et al., 1994). However, sampling programs and designs for commercial fisheries (e.g. random sampling, two-phase sampling) have not yet been ana lysed extensively in the scientific literature. The impacts of sampling variation on stock status measures are evaluated in Pelletier and Gros (1991) and Nandram et al. (1997). In Mediterranean, probably excluding some particular areas, the monitoring of fishery-dependent data is extremely difficult, due to the characteristics of fisheries briefly described in the paragraph 3. However, an effort is required for gathering this kind of information, even in light of indicators that should be adopted for sustainable development of marine capture fisheries (FAO, 1999). This issue was addressed by the

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European Commission that promoted a wide program of data collection from non-direct and direct methods (Regulation n° 1543/2000). Another and not easy goal should be the improvement of statistical systems. Fishery- independent surveys offer the best opportunity for controlling sampling conditions over time and the best choice for achieving reliable indices. A very extended literature exists world-wide on this approach. However, at least estimates of fishing effort and fishing pressure by area should be coupled with direct estimates, for making assessment and management advice more reliable. This can hold considering that management by total allowable catches is not a va lid option for most part of the Mediterranean resources. Under the precautionary approach, very conservative measures are required when there is a little scientific basis for advice. In general, “provisional reference points may be established by analogy to similar and better-known stocks”, along with “enhanced monitoring so as to enable revision of provisional reference points as improved information becomes available”. However, reference points established by analogy are not always appropriate. For examp le, Western Atlantic Canadian cod stocks have collapsed under fishing mortality rates equal to or lower than those applied to eastern Atlantic cod stocks. The reasons for such differences in resiliency have not been identified but are presumably related to population growth rates, fishery selectivity patterns in relation to fish age and maturity, and perhaps to the existence of natural refugia for specific age- or size-classes (Richards and Maguire, 1998). The question of appropriate reference points for a variable environment has received limited scientific attention to date (Thompson, 1994). If this is true for fisheries of the world where a considerable effort in data collection, improvement of methodologies, development of basic and applied research have been spent since long time, all more reason it is for the Mediterranean. Following the precautionary approach and guidelines from FAO (1995) the use of area closures is recommended to limit risks, provide refuges, and allow experimental comparisons with fished areas. Marine protected areas (MPAs) can be considered as complementary or alternative to conventional management measures that have not always prevented overexploitation of stocks, or sometimes even collapse of the fishery (Cushing, 1975). Fishing refugia may also play an important role of serving to buffer against management errors and recruitment failure (e.g. Roberts and Polunin, 1991; Walters and Parma, 1996). Modelling suggests that MPAs can function as a hedge against inevitable management limitations, thus greatly enhancing the long-term sustainable exploitation of fishery resources (Lauck et al., 1998). Moreover, MPAs are also perceived as a way to achieve conservation at smaller direct coast, however, the characteristics to make them effective tools for fishery management are the subject of current debate (Richards and Maguire, 1998). Even in the Mediterranean, where some nursery areas have been identified, approaches related to define the characteristics of MPAs and assess their effect on the fishery resources need to be developed. Any stock assessment model involves choices at two levels: the structural model that will be used and the parameter values to be adopted. Simulation studies performed in other geographical areas (AA. VV., 1998) demonstrated that assessment are sensitive to underlying structural features of fish stocks and fisheries practices, such as natural mortality, age selectivity, catch reporting and variations in these or other quantities.

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Even in the Mediterranean situation, the development of simulation studies, accounting for the outlined peculiarities, are desirable. While modelling techniques have improved, current models have been rather unsuccessful in providing suitable forecast (Sharp, 1995). In particular, outputs from a given model run should not be misinterpreted as reality. Rather, outputs from models with different structural assumptions should be compared (Richards and Maguire, 1998). This is particularly true considering the magnitude of uncertainty in the dynamic of exploited fish population, fisheries management and reference points that make the precautionary approach necessary. The issue of uncertainty is an expanding theme in fisheries management science (e.g. Francis and Shotton, 1997) that needs to be tackled in the Mediterranean fishery. In the Mediterranean fisheries, there have been up to now few attempts to consider the inherent spatial components of commercial catch data and their variability for stock assessment purposes. It is desirable that in the near future many models that address the spatial components of these fisheries will be developed for providing advice regarding the choice of an optimal effort partitioning by area, season and fishing technique, aimed at the maximisation of incomes in a precautionary context. Therefore, assessment methods that account for such heterogeneity and flexibility should be developed in the Mediterranean context, and proper applications to real data are needed. Approach accounting for the multispecies characteristics of the Mediterranean fishery need to be also implemented. Yield-per-recruit based models for the assessment of multispecies- multigear fisheries were used for the definition of which is the best combination of frequency of utilisation of competing gears and fishing strategies that optimise yields, revenues, and overall yield per recruit. Some constraints, based on some limit reference point, were introduced in order to proceed to the optimisation keeping all the involved species over a certain threshold that is considered “safe” for the self-renewal of the stocks. These simple models, however, are essentially a sum of single species yield-per-recruit analysis and assume that the recruitment of the different species remain in the same proportion over a wide range of fishing mortality. This is not likely to occur. Therefore, it should be necessary to consider, but this is very difficult in the Mediterranean, the ecosystem interactions that may produce changes in biomass, in growth and natural mortality rates, in recruitment strength related to each single species. More research effort addressed to improving knowledge on Stock/Recruitment relationships and on population’s compensatory density-dependent controls is thus necessary. The need for a new paradigm for assessment and management is now widely recognised, and this appears to have particular relevance for the situation in the Mediterranean (Caddy, 1998). “Priority should be placed on monitoring of abundance of adults and juveniles by means of regular survey, and the integration of the survey results with statistical data from the commercial fleet. These information will be essential to assessments of the stocks and later to multispecies and bioeconomic models. A control of the number of fishing operations, and/or the number of vessels having access to the fishing grounds appears to be essential for maintaining their productivity. From Mediterranean experience to date, indirect control of size captured, such as closure of the nursery grounds seems to offer the highest prospects of success, as also

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take into account the economic yield from the combination of species captured” (Caddy, 1993). These objectives of management need to be implemented on the basis of proper quantified estimates and advises from the research. Even though summarised in few words, such an approach can be actualised and formalised only if data coming from different source (e.g. from direct and indirect methods) are available and if different methods of assessment are applied. As highlighted in the previous paragraph, each of the assessment methods briefly analysed has pros and cons and is, in addition, characterised by specific assumptions, limitations and constraints. This imply that each assessment method cannot be considered self-sufficient, unless introducing sources of bias in diagnosis and advice. This consideration reflect the awareness, widely recognised in the world, of the uncertainty level related to any stock assessment. At present, the only way to make the estimates as robust as possible may rely on the adoption of different assessment strategies and on the critical comparison of the results, especially where data from different sources are already available.

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