Development and testing of fish-retention devices for pots: transparent triggers significantly increase catch efficiency for Atlantic cod (Gadus morhua)

Fish pots have lower catch efficiency than gillnets and trawls and, therefore, are rarely used for catching Atlantic cod (Gadus morhua) and similar species. Fish-retention devices (FRDs), non-return devices that permit fish to enter the pot while impeding exit, reduce the pot exit rate and therefore can increase catches. Conventional FRDs, however, also reduce entry rate and may not improve catches. To increase pot-catch efficiency, we developed and tested a new trigger-type FRD, made of transparent acrylic glass, which we named acrylic fingers (AFs). AFs are almost invisible underwater and offer little resistance to entering cod. We compared AFs with Neptune fingers (NFs), a conventional trigger-type FRD with a distinct visual outline, by observing cod entry and exit rates through both trigger types rigged to a pot in a net pen. Both trigger types significantly reduced exit rates compared with a funnel without triggers; however, NFs also reduced entry rates by visually deterring cod. Specifically, AFs have higher entry-to-exit ratios and therefore improve catch efficiency. Combining AFs with funnels further increased catch efficiency. Thus, transparent acrylic triggers present a promising new approach to increasing pot-catch efficiency and may increase the uptake of the cod pot, an environmentally low-impact gear.