Longline selectivity and biology of 'red' sea breams (Sparidae) - 1995-1996. DG XIV, Ref. 94/059

Summary for non-specialists

Size selectivity of hooks in a Portuguese longline fishery targetting "red" sea breams (Pagellus acarne and Pagellus erythrinus) was studied. Experimental longlining was carried out with 3 hook sizes (numbers 11, 13 and 15; MUSTAD brand, round bent, spade end, Quality 2316 DT) and two baits (razor shell and ghost shrimp). A total of 33600 hooks were fished in 28 fishing trips. Pagellus acarne and Pagellus erythrinus dominated the catches along with two other sea breams (Sparidae): Diplodus vulgaris and Spondyliosoma cantharus. Species diversity was lower than in the previous "white" sea bream longline selectivity study (BIOECO/93/004), but catch rates were much higher; reaching 20-30% (number of fish per 100 hooks), with total catches of up to 60 kg per 1200 hook set.

A new methodology was used to fit logistic and normal selectivity curves to the catch size frequency distributions for the four species. The catch size frequency distributions for the 3 hook sizes were generally highly overlapped. While the logistic selectivity curve was found to be more appropriate for D. vulgaris, P. erythrinus and S. cantharus, a better fit was obtained with the normal selectivity curve for P. acarne.

All three hook sizes caught a wide range of size ranges for all species. In general the smallest hook (number 15) had the highest catch rate. However, for P. acarne the intermediate sized number 13 hook was clearly the most successful in terms of catch rate, while no clear differences in catch rates were found for P. erythrinus.

Few illegal sized fish were caught, even though the smallest sized hook is used in recreational fishing to catch sea breams of less than 15 cm total length. The lack of small individuals in the catches may be due to competition for the baited hooks with larger individuals or because fishing took place at depths from 45 to 55 m and 6 to 9 km offshore and juveniles and smaller individuals of these species tend to be found more inshore or even in lagoons and estuaries.

The type of bait did not have a significant effect on the catch size frequency distributions of the two main species for the three different hook sizes used. However, bait type did affect the catch rates with more fish caught overall with razor shell while ghost shrimp was more effective for the two "red" sea breams. The time of setting the longline was found to be critical, with the highest catch rates obtained for sets made less than one hour before sunrise. Sets made after sunrise caught no P. acarne. Catch rates also differed significantly between the two fishing grounds which were approximately 40 km apart.

A comparison of the catch size frequency distributions of fish caught by longline with those caught by monofilament gill nets showed that longlines generally caught larger fish and were less selective in terms of the size range; with a wider range of sizes caught.

Biological studies of the two "red" sea breams were carried out. Age and growth studies using otoliths allowed the construction of age-length keys for both species and the estimation of growth parameters for P. acarne. Estimation of growth parameters for P. erythrinus was not possible due to insufficient sample size. The large number of age classes for this species (>20) implies a much greater sample size than was possible in this study.

The reproduction of P. acarne and P. erythrinus was studied over a one year period. The mean monthly gonad-somatic index was calculated, along with the sex ratio, and fish were classified according to male and female macroscopic maturity stages. This work allowed the description of the annual spawning cycle for each species. Both species were found to have an extensive spawning period (spring to autumn), with a winter resting period.

Stomach content analysis was carried out in order to evaluate the feeding habits and diets of the two "red" sea breams. Both species were found to be opportunistic; feeding mainly on a wide range of small benthic invertebrates.

This study is the second on longline "metiers" of the south of Portugal. The longline is a static gear which has received little attention, particularly in southern European waters. However, regionally it may be an important gear. In the Algarve for example a significant proportion of all commercial fishing licences are for longline gear. This study has shown that in this fishery for "red" sea breams, the longline is a highly viable specialist gear, with excellent catches of high value species.

From economic and conservation points of view it is a gear which is highly recommended. There is little or no waste in terms of discards, a high value quality product is landed, fuel costs are relatively low, and capture of juvenile or illegal sized fish is minimal.

We believe these results are of relevance to other longline fisheries within the European Union, particularly within the Mediterranean basin where similar gears are used to fish the same or closely related species. The information obtained will contribute to the better understanding and management of complex multi-species and multi-gear fisheries.

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Erzini, K., Gonçalves, J.M.S., Bentes, L., Lino, P.G. and Ribeiro, J. (1997) Longline selectivity and biology of "red" sea breams (Sparidae). Ref. 94/059. Final Report. European Commission, DG XIV/C/1, Universidade do Algarve, UCTRA, Faro, 100pp + annexes