The Atlantic cod was once the most important fish to commercial fisheries in New England. Intense commercial fishing pressure has led to severe population crashes in recent years. As a result, the commercial Atlantic cod fishery is now much reduced from historic levels.
At present, our scientists are part of a collaboration examining new ways to study discard survivability in this species. In the past, we conducted field-based investigation on the impacts of capture on the Atlantic cod. We worked with New England commercial longline fishermen to determine the impact of capture on undersized Atlantic, and their subsequent chance for survival post-release.
Cod captured on commercial longlines are either removed mechanically or by hand. Careful removal of the hook by hand left a small hole in the fish’s jaw, while mechanical removal generally resulted in either one or both jaws broken. We compared survival rates by holding the fish in large cages at their capture depth for 72 hours, and found that hand-removed fish were twice as likely to survive 72 hours than mechanically separated fish.
We worked with participating fishermen to develop a less damaging hook removal method in order to improve the survival of the sub-legal sized bycatch. Participating fishermen installed a foot pedal mechanism to retrieve their longline gear that left their hands free to handle the fish. A maneuver that flipped the fish around the circle hook left a small hole in the jaw similar to when hooks were carefully removed.
Preliminary results using this less traumatic method of hook removal did improve the survival of juvenile cod bycatch over mechanical release however sample size was too small to allow robust statistical treatment.
Biochemical analyses revealed that all cod showed significant changes in their blood profiles after being removed from longline gear. Seventy-two hours later, most parameters had either recovered to normal values or were not significantly different than control values.
However, potassium ions were significantly lower than normal after 72 hours in those fish that were mechanically removed from the longlines. The low potassium ion concentrations suggest that hemorrhaging from the more severe injuries may have contributed to the poor survival. Unfortunately further experimentation was inconclusive concerning the relationship between potassium ion concentrations and survival.