Effects of hook size on the catch variation, size selectivity, and bycatch of the bottom set longline fisheries targeting nemipterid fishes in the Philippines

In the Philippines, nemipterid fishes hold substantial economic importance due to their high market value. However, the local longline fisheries lack specific regulations for managing the capture of these fishes. This study aimed to determine the optimum hook size for increasing catchability in nemipterid fishing while minimizing bycatch, thus promoting sustainable fisheries. We conducted experimental fishing at three depth ranges, using Mustad J-type hooks of 11 different sizes, ranging from #562, the largest, to #572, the smallest. All Mustad J-type hooks utilized medium-sundried Sardinella lemuru as bait. Seven Nemipteridae species and 31 other bycatch species were collected, with Nemipterus nematophorus emerging as the predominant species, constituting 61.96% and 56.99% of the catch by count and weight, respectively. The largest hook size (#562) demonstrated the lowest catch in both fish count and weight, revealing its inefficacy in capturing a wider size variation of fish. However, larger fish were collected with larger hooks, as suggested by the generalized additive model (GAM). In contrast, the generalized additive model suggested that middle-sized hooks (#566 to #568) yielded the highest catches but with an increased bycatch. Thus, these hook sizes, commonly used by many fishermen, were identified as the most exploitative and risky in terms of increasing bycatch. Using large hooks, preferably #562 to #565, would lead to an increase in the individual catch size for nemipterids, to reduce the small-size fish, and to minimize bycatch. These results suggest that regulating hook sizes is essential for sustainable fishing, which can result in a potential short-term reduction in catch rates but offer long-term benefits for nemipterid fisheries management.