Species and size selectivity in a Portuguese multispecies artisanal long-line fishery

Karim Erzini ] 0 Jorge M. S. Gonalves 0 Luis Bentes 0 Pedro G. Lino 0 Jo ao Cruz 0 0 K. Erzini: Unidade de Ciencias e Tecnologias dos Recursos Aquaticos (UCTRA), Universidade do Algarve , 8000 Faro , Portugal [ The species and size selectivity of long-lines using small hooks were studied oV the south coast of Portugal using ''Mustad'' brand round bent, flatted sea hooks (Quality 2316 DT) numbers 15, 13, and 11 baited with razor shell clam (Ensis siliqua). Hook numbers 13 and 11 are 49 and 109% larger respectively than number 15 hooks in terms of overall size (maximum width#maximum length). A total of 39 900 hooks were fished in 45 sets and 35 species of fish and cephalopods were caught. As a group, 13 species of sea breams (Sparidae) dominated the catch by numbers (58%) and weight (73%). Six species of sea breams, along with the greater weever fish (Trachinus draco) accounted for 81% of the total catch by weight, with the common or white sea bream (Diplodus sargus) being the most important (29%). Catch size distributions by hook size were, in general, highly overlapped for all species and hook size had little apparent eVect on minimum size at capture. All hooks caught a wide range of sizes per species, but the catch rate (number of fish per 100 hooks) was significantly lower for the largest hook. Except for the black sea bream (Spondyliosoma cantharus), capture of illegally sized or immature fish was minimal. Small increases in average size with hook size were evident for four species: Diplodus sargus, D. vulgaris, Lithognathus mormyrus and Serranus cabrilla. No diVerences in size selectivity were detected for Boops boops, D. annularis, Spondyliosoma cantharus and Trachinus draco. A skew-normal model adequately described diVerences in size selectivity in five of six species. Introduction Artisanal fisheries are of considerable importance in Portugal, involving 84% of the fishing fleet (12 496 of 14 866 boats (G.E.P.P., 1992)) and employing a large number of fishermen. Along the south and south-west coasts small boats account for up to 50% by weight and 90% by value of the total landings. These boats capture high-value species using long-lines, gillnets, trammel nets, purse seines, traps, and pots. Long-lines with small hooks are widely used to capture a variety of species at depths ranging from 10 to more than 100 m. With hook numbers from 1 (largest) to 20 (smallest), the hooks used are at the lower end of the size range, with the number 13 being the most widely used. A long-line consists of a monofilament mainline (usually 1.01.2 mm diameter) with 0.5 or 0.6 mm diameter monofilament gangions of variable length (0.71.0 m) spaced 1.53 m apart. The long-line is coiled in round plastic tubs with cork rims for fixing the hooks. Each tub may contain 300 or more hooks. Layers of damp beach sand are used to keep the long-line properly coiled. The most commonly used hook design on the south and south-west coast of Portugal are Mustad round bent, flatted sea hooks. The hooks are baited before the fishing operation with ghost shrimp, clam, squid, cuttlefish or sardine pieces/ strips and are left hanging loosely over the side of the tub. The long-line is fished on the bottom and is weighted at both ends with a rock or lead weight attached to a buoy. Depending on the length of the mainline, additional buoys and weights may be attached at intervals. Deployment of the gear involves raising the tub on a platform to avoid snagging the side of the boat or the engine, attaching the end of the long-line to the weight, sending the weight to the bottom, and then using the momentum of the moving boat to pay out the long-line. The fisherman is limited to rotating the tub, occasionally untangling hooks, linking the end of a long-line to the next tub and adding weights and buoys. In this way a typical artisanal inshore fishing boat (56 m with an outboard engine) with two fishermen may fish a long-line with 4000 or more hooks, with one man handling the engine while the other sets the gear. However, some fishermen fish alone and can singlehandedly set and retrieve their long-line. The long-line is retrieved manually as the boat moves between buoys. It is placed in the tubs and is repaired and re-coiled properly on land. A landing net is used for the larger fish. The time of setting the long-line and the duration of the set depends on the species sought, area, depth and bait used. The target species are principally high value sea breams (Sparidae): Pagrus pagrus, Pagellus acarne, P. erythrinus, Diplodus sargus, D. vulgaris, D. cervinus, D. puntazzo, Sparus aurata, Lithognathus mormyrus and Spondyliosoma cantharus. Although long-lines using small hooks are widely used in many countries, there have been few studies to date concerning this gear. This work is part of an ongoing study on the artisanal, small scale fisheries of the south and south-west coast of Portugal which includes aspects of fisheries biology, ecology, gear selectivity and population dynamics of the more important species. The primary objective of this study was to evaluate species and size selectivity in a multispecies artisanal long-line fishery in Algarve waters (south of Portugal). Methods A preliminary survey of local (Algarve) artisanal fishing practices was carried out in order to select the hook sizes, bait type and gear specifications to be used in experimental fishing. The most commonly used hook size was number 13. It was decided to carry out the selectivity studies with hook numbers 15, 13, and 11 (i.e. two hook sizes above and below number 13). The dimensions of these round bent, flatted sea hooks (Mustad brand Quality 2316 DT) are given in Figure 1. Using maximum width multiplied by maximum length to represent overall size (Otway and Craig, 1993), hooks number 13 and 11 are 1.49 and 2.09 times the size of the number 15 hook, respectively. The mainline and the gangions were 1.1 and 0.5 mm diameter monofilament and the bait chosen for the selectivity studies was razor shell clam (Ensis siliqua). A 600 hook long-line was constructed (75 cm gangions, attached directly to the mainline, and spaced 1.5 m apart), with 200 hooks of each size in a series. After consultation with local fishermen it was decided to fish within a restricted area and depth range since species composition changes with depth. Consequently it was decided to concentrate on the white sea Hook 15 Hook 13 Hook 11 7.63 0.07 8.75 0.06 10.53 0.04 8.23 0.03 9.93 0.04 11.89 0.05 breams (Sparidae), namely Diplodus sargus, D. vulgaris, D. cervinus, D. annularis, Spondyliosoma cantharus, Lithognathus mormyrus, and Sparus aurata which are fished year-round in relatively shallow water (less than 30 m). At greater depths red sea breams such as Pagrus pagrus, Pagellus acarne, and P. erythrinus are the main species. The area selected was oV the Praia de Faro (36)59*N, 8)00*W) and consists of a sandy bottom with patches of rocks and gravel at depths b (...truncated)