Biology

Understanding population dynamics of fish stocks is essential in developing optimal fisheries management strategies. This is often obtained through fitting mathematical models to information/data collected from the fisheries to estimate vital fisheries parameters and their uncertainties. The quantity of fisheries information, often positively related to economic and social values of fisheries, is one of the most important factors influencing the quality of fisheries parameter estimation. We classify the data quantity into two categories: diversity of the information, defined as the number of sources from which the information about a fisheries variable is collected, and Aquat. Sci. 65 (2003) 1-07 Aquatic Sciences number of observations made for a given fisheries variable. Using an abalone fishery as an example, we demonstrate the importance of data quantity in stock assessment and management. Deficiency in data quantity tends to yield biased assessment of the status of fisheries stock and increase the uncertainty in stock assessment, subsequently complicating the identification of an optimal management strategy. This study suggests that there is a need to determine the relative importance of different types of fisheries data for stock assessment in allocating sampling effort to ensure that the most critical information is collected.

Intraspecific variations in size-and age-at-maturity were studied in red bandfish, Cepola macrophthalma, in two adjacent gulfs of the western Aegean Sea, in the southern of which the population of red bandfish is stunted. Samples were collected with a commercial trawler over a grid of 34 stations at depths ranging from 22 to 222 m. The hypothesis tested was that length and age at 50% maturity, L m50 and t m50 respectively, for males and females do not differ in the two regions. The results showed that the L m50 of both males and females in the northern area was by 3.5 cm larger than that in the southern area and the 95% confidence intervals of L m50 in the two areas did not overlap. Although the t m50 of males was larger in the northern area, the 95% confidence intervals of t m50 overlapped in the two areas whereas for females, the t m50 was larger by 0.4 years in the northern area and the 95% confidence intervals of t m50 in the two areas did not overlap. Stunting of the red bandfish growth in the southern area is the result of the combination of an extremely low food availability with higher temperatures prevailing in that area. Implications of these fine spatial scale intraspecific differences for the fisheries management of the highly oligotrophic eastern Mediterranean Sea are also discussed.

We identified and quantified the effect of season, depth, and inner and outer panel mesh size on the trammel net catch species composition and catch rates in four southern European areas (Northeast Atlantic: Basque Country, Spain; Algarve, Portugal; Gulf of Cádiz, Spain; Mediterranean: Cyclades, Greece), all of which are characterised by important trammel net fisheries. In each area, we conducted, in 1999-2000, seasonal, experimental fishing trials at various depths with trammel nets of six different inner/outer panel mesh combinations (i.e., two large outer panel meshes and three small inner panel meshes). Overall, our study covered some of the most commonly used inner panel mesh sizes, ranging from 40 to 140 mm (stretched). We analysed the species composition and catch rates of the different inner/outer panel combinations with regression, multivariate analysis (cluster analysis and multidimensional scaling) and other 'community' techniques (number of species, dominance curves). All our analyses indicated that the outer panel mesh sizes used in the present study did not significantly affect the catch characteristics in terms of number of species, catch rates and species composition. Multivariate analyses and seasonal dominance plots indicated that in Basque, Algarve and Cyclades waters, where sampling covered wide depth ranges, both season and depth strongly affected catch species compositions. For the Gulf of Cádiz, where sampling was restricted to depths 10-30 m, season was the only factor affecting catch species composition and thus group formation. In contrast, the inner panel mesh size did not generally affect multidimensional group formation in all areas but affected the dominance of the species caught in the Algarve and the Gulf of Cádiz. Multivariate analyses also revealed 11 different métiers (i.e., season-depth-species-inner panel mesh size combinations) in the four areas. This clearly indicated the existence of trammel net 'hot spots', which represent essential habitats (e.g., spawning, nursery or wintering grounds) of the life history of the targeted and associated species. The number of specimens caught declined significantly with inner panel mesh size in all areas. We attributed this to the exponential decline in abundance with size, both within-and between-species. In contrast, the number of species caught in each area was not related to the inner mesh size. This was unexpected and might be a consequence of the wide size-selective range of trammel nets.

We estimated the relationships between total length (TL) and body girth (a) across the vertical eye diameter (G1), (b) behind the gill-cover (G2) and (c) in front of the first dorsal fin (G3), for 18 marine fish species from Cyclades waters (Aegean Sea). G1, G2 and G3 increased linearly with TL for 12, 13 and 12 species, respectively, and loglinearly for the remaining species. All r 2 values were statistically significant (>0.61, P < 0.05). When G1, G2 and G3 were plotted against TL for all 18 species combined, three general length-girth relationships were identified, the slopes of which differed significantly (ANCOVA, P < 0.05). These relationships generally corresponded to three general body shapes. The implications of girth measurements for selectivity estimates are also discussed.