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Fishing is associated with increased reproductive investment in cod - but not quite as expected

A new study shows that reproductive investment in cod increased during a period of heavy exploitation, and then stabilized or possibly reversed when the exploitation pressure was greatly reduced. However, this pattern was only found for males.

Torsk
Torsk (Gadus morhua)

Life-history theory predicts that increased mortality favours increased allocation of energy into reproduction. Earlier maturation is one way of achieving this, and the fisheries-induced adaptive change that has been most actively studied (see our review). However, it is equally expected that the energy allocated to reproduction during any reproductive cycle should also increase. This prediction is much less tested, largely because suitable data are scarce. Luckily, the Fisheries and Oceans Canada (DFO) at St. John's has been collecting gonad weight data as part of their regular sampling of Atlantic cod for decades. With EvoFish alumnus Loïc Baulier in lead, we analysed these data to test whether reproductive investment—here estimated as gonad weight relative to body weight—increased in Atlantic cod off Newfoundland during the period of heavy exploitation that eventually lead to collapses of these stocks, and whether the increaing trend stabilized or reversed when the exploitation pressure was greatly reduced during the moratoria declared after the collapses?

Much to our surprise, we found the predicted patterns in male cod but not for female cod. A priori, we expected the patterns to be clearer for female cod, because of the more direct link between gonad weight and reproductive success in female compared to male cod. A possible explanation for this unexpected result is that gonad weight varies much more in females during the spawning season than in males, making gonad weight a poor indicator of the actual energetic investment (hydrating oocytes makes them heavier but is not increasing their energetic content).

The study has been published in FACETS, the new open access journal published by the Canadian Science Publishing, a respected publisher.

Reference:

Baulier, L., M. J. Morgan, G. R. Lilly, U. Dieckmann, and M. Heino. 2017. Reproductive investment in Atlantic cod off Newfoundland: Contrasting trends between males and females. FACETS 2:660–681. doi: 10.1139/facets-2017-0005.