Effects of a codend mesh size increase on size selectivity and catch rates in a small-mesh bottom trawl fishery for longfin inshore squid, Loligo pealeii

Abstract

Loligo pealeii (longfin inshore squid) co-occurs with Atlantic butterfish ( Peprilus triacanthus) throughout the year and discarding in the L. pealeii bottom trawl fishery is the primary source of fishing mortality on the butterfish stock. Consequently, a codend mesh size increase in the Loligo fishery has been proposed as a management measure to minimize discarding of butterfish and other bycatch species. A paired-tow study was conducted using a Loligo twin trawl to assess the effects of a codend mesh size increase, from 50 mm to 65 mm (inside stretched mesh), on catch rates and size selection of the target and bycatch species. Relative mesh selection factors estimated from a SELECT model were: 1.7, 1.5, 2.2, and 3.0 for L. pealeii; P. triacanthus; Illex illecebrosus (Northern shortfin squid); and Merluccius bilinearis (silver hake), respectively. Catches of butterfish and silver hake in the 65 mm codend were reduced by 58% and 41% in terms of numbers, respectively. However, a larger mesh size would be necessary to allow 50% escapement of the median sizes of mature silver hake and butterfish. A trade-off associated with the bycatch reductions is a 29% loss in the catch weight of the target species. However, the reduction in ex-vessel value of Loligo catch is probably not proportional to the percent loss in Loligo catch because most of the loss consisted of squid from the smallest market size categories which have the least value and are primarily discarded. In addition, a greater percentage of large, more valuable squid was caught in the 65 mm codend. The September study results likely represent a worst-case scenario with respect to Loligo catch loss because the monthly mean body size of Loligo tends to be smallest during September. Most catches of the three bycatch species evaluated herein and Loligo smaller than 10 cm are discarded, most likely dead, in the Loligo fishery. Therefore, a codend mesh size increase to 65 mm should provide some conservation benefits to these stocks if a portion survive escapement. On a fleet-wide basis, the magnitude of bycatch reductions and Loligo catch loss will vary depending on seasonal changes in mean body size, vessel-specific gear characteristics and fishing practices.