Studies of the discards of commercial fisheries from the South coast of Portugal - 1996-1998. DG XIV, Ref. 95/081

Summary for non-specialists

In this study we felt it was necessary to define "retained catch" because of the observation that the species (quantified and/or identified) which reached the fishing boat deck and were not discarded, sometimes were not the same as the species landed. In addition, it was often not possible to obtain the official landing records of specific boats (landed catch).

The assessment of by-catches and discards in coastal fisheries has important biological and socio-economic considerations in the management of fisheries. Discards represent a loss in terms of production. This is particularly important when discards consist largely of juveniles of marketable species. The by-catch of juveniles may have a significant impact on recruitment to the commercial fishery. Discards of non-marketable species may represent losses in economic opportunities in cases where these could be used in the production of fish meal, fish oils, fish pastes, or other products.

The effects of the discarding of by-catch at sea on community structure, trophic interactions, and stability are poorly known. However, there is some evidence that large quantities of discards may have a significant impact: on multispecies fisheries. With the increasing emphasis on multispecies or ecosystem-based management approaches, it is necessary to assess and quantify discard composition and mortality. The by-catch of rare and/or endangered species, particularly marine mammals, sea birds, and reptiles is an increasingly sensitive and important area, with implications for environmental impact, fisheries management and regulation.

An analysis of the literature has shown that discarding is an important problem Within the EC there is a great variation due to type of fishery, type of gear and socio-economic aspects. In Portugal, coastal fisheries employ a great variety of techniques ranging from trawls to artisanal gears. The by-catches and discards of many of these fishing gears have not yet been studied in terms of composition, quantity, causes, possible impacts, and management implications. This is the first study on discards in Portuguese waters.

Since by definition discards are never landed, it was necessary to have observers on board the fishing boats. This methodology allowed us to get extra information, important not only for the statistical analyses, but also for the understanding of the reasons for discarding.

One or two observers; went aboard every fishing boat sampled. Most of the fishing boats sampled were quite small, with limited space for the observers to work. To avoid any conflict with the work of the fishermen and to be able to do as much as possible with the discarded, species, it was decided to bring the discards to the laboratory for all type of characterization. In the case of relatively small amounts of discards, the entire catch of discards was brought back to the laboratory. When the quantity of discards was large, sub-samples were taken (15-30 kg) and estimates of the total amount of discards were made on board. In this way, the composition of the discard catch can be made based on the sub-sample.

All species caught were classified. by their frequency of rejection on a three point scale: 1 -occasional (species only occasionally are discarded), 2 - frequent (species frequently discarded), 3 - regular (species always discarded).

During the sampling period of the study, from March 1996 to June 1997, a total of 24 fishing vessels were sampled (6 crustacean trawls, 6 demersal purse-seiners, 3 fish trawls, 3 pelagic purse-seiners; and 6 trammel nets), with a total of 57 fishing trips and 128 hauls/sets.

A total of 236 species were identified from all taxa, and there all were classified by the frequency of rejection and their occurrence (number of trips in which the species has occurred) in the métiers studied was registered. Most species were discarded (139) followed by species frequently discarded. (79). Very few were species occasionally discarded (18).

CRUSTACEAN TRAWL

Due to the target species being crustaceans, crustacean trawls (CT) fishes in deep waters, (more than 200 meters, at average depths of 400-500 meters).

The fish species which occurred most frequently in the CT sampled were, Conger conger (conger eel) and Micromesistius poutassou (blue whiting), while the invertebrate species were the cephalopods Illex coindetii (broadtail squid), Todaropsis eblanae (lesser flying squid) and Eledone cirrhosa (curled octopus). In terms of total amounts discarded (in weight), the most important species were: the dark electric ray Torpedo nobiliana (14.8%), the dogfish Scyliorhinus canicula (14.6%), the conger eel Conger conger (10.4%), the boar fish Capros aper (6.0%), the hake Merluccius merIuccius (5.5%), the broadtail squid Illex coindetii (5.1%) and the blue whiting Micromesistius poutassou (4.8%).

The discard ratio for the CT was between 36% and 91%, with an average of 83%, during the study period.

DEMERSAL PURSE SEINE

The demersal purse seine (DPS), has the same characteristics as the pelagic purse seine (PPS). The main difference is the size of the vessels: DPS are smaller fishing boats than PPS. The size of the DPS boats sampled were between 13 and 17 meters, while the PPS boats sampled were around 21 meters. Also, the difference is that DPS fish at shallower depths, near the coast, while PPS fishes at greater depths. The main target species of the PPS is sardine, while for DPS is a mixture of species. DPS fishing vessels are prepared to be able to separate the fish on board, while PPS do not separate the catch.

The fish species which occurred most frequently in DPS were Sardina pilchardus (European pilchard), Scomber japonicus (chub mackerel) and Boops boops (bogue). However, in terms of quantities discarded the grey mullet Liza aurata was the most important species with 66.2% of the total amount discarded, followed by Scomber japonicus (24.4%), Boops boops (2.9%) and Sardina pilchardus (2.6%).

The discard ratios for the DPS were between 1 % and 86%, with an average of 3 5%, during the study period.

FISH TRAWL

Several species of fish occurred significantly: Capros aper (boar fish), Citharus linguatula (spotted flounder), Scomber japonicus (chub mackerel), Serranus hepatus (brown comber), and others. In terms of total amounts discarded (in weight), the most important species were the snipefish Macrorhamphosus scolopax (37.9%), the chub mackerel Scomber japonicus (17.0%), the boar fish Capros aper (14.8%), the American John Dory Zenopsis conchifer (5.0%) and the bogue Boops boops (5.2%).

The discard ratio for the FT was between 59% and 91%, with an average of 79%, during the study period.

PELAGIC PURSE SEINE

The species of fish which occurred most frequently were Sardina pilchardus (European pilchard) and Scomber japonicus (chub mackerel). The most important species in terms of quantities discarded were the bogue Boops boops (31.3%), the chub mackerel Scomber japonicus (17.9%), the garfish Belone belone (15.5%), the European pilchard Sardine pilchardus (14.8%), the snipefish Macrorhamphosus scolopax (10.7%), and the Atlantic mackerel Scomber scombrus (5.5%).

The discard ratio for the PPS varied between 1 % and 94%, with an average of 60%, during the study period.

TRAMMEL NET

The species of fish which occurred more often were Solea spp. (soles), seabreams (Sparidae) and Scomber japonicus (chub mackerel). However, in terms of biomass discarded, the most important species, was the chub mackerel Scomber japonicus (57.8%), followed by the European pilchard Sardina pilchardus (11. 5%) and the snipefish Macrorhamphosus scolopax (8.1 %).

The discard ratios for the TN varied between 2% and 29%, with an average of 9%, during the study period.

DISCARDS ESTIMATES

The estimation of the total discards based on official landing statistics shows values for 1996 of 35 475 tonnes for the trawl fisheries, and 39 994 tonnes for the purse-seines. For the first three months of the year 1997, the values are around 1 508 tonnes for the trawls, and 1 811 tonnes for the purse-seine.

REASONS FOR DISCARDS

The reasons for discarding were mainly of low market value (e.g. the mackerels), no readily available market (e.g. snipefish) and damaged or poor quality fish (e.g. hake and common squid).

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Anon. (1997) Studies of the discards of commercial fisheries from the South coast of Portugal. Final report. DG XIV, Project 95/081. 30pp + annexes.