Experimental By-catch Reducing Devices (BRD) in the demersal purse-seine fishery and evaluation of survivorship- 2002-2004. FCT - POCTI/BSE/43113/2001

Summary

Fisheries by-catches and discards constitute a significant problem in many fisheries worldwide. Studies off the south coast of Portugal (Algarve) have shown that the mean discard rate of purse seiners varies between 20 and 30% of the total catch. Considering the relative importance of purse seining for landings, it is estimated that just for the Algarve, purse seine discards represent some 5000 to 10000t per year. Little is known of the impact of these discards on the marine ecosystem, or even on species or populations.

The impact of by-catch and discards of fish below the minimum legal size or of species of no commercial value could be minimized if survival after discarding were high or if by-catch could be reduced. Recently, significant progress has been made in the use of by-catch reduction devices for different fishing gear. While a number of different devices have been used in trawl fisheries, there have been few applications in coastal purse seine fisheries.

The main objective of this study was to evaluate the efficacy of different selectivity devices in reducing by-catch and consequently of discards in the Algarve demersal purse seine fishery. While the demersal purse seine may have the same target species as the pelagic purse seine, it can also target high commercial value demersal species such as sea breams (Diplodus spp.) and the European sea bass (Dicentrarchus labrax). In this case it is largely the low value pelagic species such as chub mackerel and sparid juveniles that are discarded.

The study consisted of the experimental deployment of a selectivity device: the use of a panel of diamond-shaped mesh netting of 70mm, in the posterior part of the purse seine, known as the “cuba de vante”. As the net is pursed, the catch is concentrated near the fishing vessel and the pelagic species, along with small-sized individuals of other species are forced against the device and are thus able to pass through and escape. An 18 mm mesh cover was used to evaluate the selectivity of the by-catch reducing device.

The condition and survival of fish that passed through the selectivity device were evaluated. Individuals of the main discard species were collected during normal fishing operations and maintained alive in tanks on board. After an adaptation period in 500 and 1000l tanks, contact with the selectivity device was simulated. For the sardine (Sardina pilchardus), the most important species in economic terms, stress was studied by cortisol radioimmunoassay, loss of scales and long-term mortality.

The experimental fishing trial data allowed the evaluation of discards, with the chub mackerel (Scomber japonicus), the bogue (Boops boops), the sardine (Sardina pilchardus), Senegal seabream (Diplodus bellottii) and the needle fish (Belone belone) being as expected the main discard species, accounting for approximately 50.5% of the total catch. The causes for discarding were also identified, with low commercial value being the most important reason.

The results of the trials with the selectivity device or BRD, that consisted of the use of the 70 mm diamond mesh “cuba de vante”, were promising, with 61.8% of the fish escaping, especially those that are most discarded (chub mackerel, bogue, sardine, Senegal seabream and needle fish).

The study of survival showed that in general, the BRD did not have a significant effect on survival rate or on physical condition. The demersal species such as two-banded sea bream (Diplodus vulgaris), common sea bream (D. sargus), black sea bream (Spondyliosoma cantharus), sea bass (Dicentrarchus labrax) and mullet (Chelon labrosus) had higher survival rates than the small pelagics such as chub mackerel and sardine.

The cortisol analysis constituted a preliminary step in the evaluation of stress related to small pelagics capture and handling. The sardine cortisol values were very high compared to those for the demersal species, with an increase related to the amount of induced stress. Nevertheless, the simulation of passage through the BRD did not have detectable effects, suggesting that the effect of passage through the BRD does not have a worse effect than simple handling.

The selectivity device used in this study proved to be efficient in the reduction of by-catches and discards in the demersal purse seine fishery, with an increase in the catch of larger fishes. It therefore has significant benefits for the demersal purse seine fishery and possibly for other “métiers” as well. This system can be compared to the act of lowering the purse seine net which is done to reduce the catch of pelagic fishes (quota related) and can even be considered as an alternative, since it can be used earlier, therefore reducing the effect of panic, stress, loss of scales and consequently the mortality associated with the concentration of fish in the final stages of pursing the net.

Overall, the use of this method for reducing discards can be considered positive for the following reasons: there is no need for structural modification of the fishing gear, the BRD is easy to deploy, it is efficient in terms of species, sizes and quantities of fish that manage to escape and post-escapement survival is high. In addition to the effectiveness of the device tested, there are other advantages associated with the use of the BRD: