Effects of increased mesh size on catch and fishing power of coral reef fish traps

Effects of increased mesh size on catch and fishing power of coral reef fish traps (Antillean design) were investigated in the Barbados west coast trap fishery by experimental fishing with commercial traps (maximum aperture 4.1 cm) and large mesh traps (maximum aperture 5.5 cm). Large mesh traps caught 53-63% less fish by number and 51% less by weight than the commercial traps. The fish in the commercial traps were significantly smaller by length, body depth and weight than those in large mesh traps, and a significantly higher percentage were immature. The effect of mesh size on fishing power of traps was investigated by comparing the catch rates of fish large enough to be retained by both trap types (i.e., body depths >5.5 cm; termed adjusted catch). The adjusted catch of large mesh traps was 24-35% lower by number and about 30% lower by weight than that of commercial traps, indicating that the fishing power of large mesh traps is substantially lower than that of commercial traps. The squeezability hypothesis and the visual image hypothesis were tested as explanations for the reduced fishing power of large mesh traps by comparing catches of commercial traps, large mesh traps and experimental traps (where experimental traps were designed to have a similar visual image as commercial traps but similar fish retention capacity as large mesh traps). The higher fishing power of commercial traps is generated primarily by a difference in catch rates in the 5.5-6.0 cm body depth size class; i.e., the size class which might feasibly squeeze through the 5.5 cm maximum aperture of large mesh traps. This strongly supports the squeezability hypothesis as an explanation for the higher fishing power of small mesh traps. We could find no definitive evidence indicating that reduced visual image of traps, whether created by structural differences (trap construction) or biotic differences (number of fish already in a trap), decreases ingress rates to traps and hence explains the lower fishing power of large mesh traps. These results will facilitate the incorporation of the reduced fishing power effect into yield per recruit models which can be used to predict catch rate changes in fisheries in which the minimum trap mesh size is increased. (C) 1999 Elsevier Science B.V. All rights reserved.