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Selective fishing doesn’t change just the population abundances, it also changes the fish life histories

Atlantic cod was fished already thousands of years ago in the Mesolithic Stone Age. Centuries later, seafaring Vikings would dry cod in the open Nordic air and use it as the main protein in their naval explorations. In the 16th century, cod would become one of the most traded commodities during the growing economic European influence. Eventually, the abundance of a fish that was once so numerous that even Thomas H. Huxley believed its fishery to be inexhaustible, would suffer a collapse on both sides of the Atlantic, leaving some stocks, in critical need of recovery to this day.

Scientists have observed that highly selective fisheries practices, which non randomly remove the big and consequently especially the older fish, have altered not just the population abundance, but also the diversity of fish life histories, leaving populations to constitute of individuals that mature at younger ages and smaller sizes. Despite stricter and more careful measurements, including complete closers of certain fishing areas, the recovery in rebuilding the population abundances as well as fish life histories remains exceptionally slow in several stocks, and some of the mechanisms, that slow this recovery, still need to be uncovered.

Human-induced selection takes what the natural selection favors

In this thesis, I explored the evolutionary and ecological advantages that the life history diversity of the iconic Atlantic cod, Gadus morhua, generates and maintains at different levels of biological organization. My contribution was applying a mechanistic modelling approach to identify whether a body size-related multiple-batch spawning strategy, where cod spawns the eggs in several batches during an extended spawning season, constitutes a bet-hedging, and if this may be one of the mechanisms contributing to a slow recovery of overfished populations.

“By observing long-run fitness under different simulated environmental conditions, we found that multiple-batch spawning is a bet-hedging strategy which increases the chances of survival for the offspring of cod when environmental conditions are unpredictable and dangerous.”

But while the strategy provides risk-spreading benefits under the natural selection, it not only fails to protect the population against size-selective fishing but also makes the population more vulnerable to such human-induced selection.

Changes within species can affect the whole food web structure and functioning

In the second part of the thesis, I tackled the role of intraspecific diversity in the food web structure and functioning. Ecological differences that occur within species can be in some cases comparable to the ones that occur among species. Two cod ecotypes, the Fjord cod and the North Sea cod, co-exist in Skagerrak. The ecotypes differ in their life-history traits and ontogenetic dietary shifts. This raises a rather applicative question of whether the scientific advice and the fisheries management should account for the distinctive role of the ecotypes when planning the total allowable catches for cod.

“Our ecosystem model showed that the loss of the smaller Fjord cod from the Skagerrak food web could lead to a greater predatory pressure of the North Sea cod on other valuable commercial species such as whiting (Merlangius merlangus), herring (Clupea harengus) and mackerel (Scomber scombrus).”

We found that maintaining intraspecific diversity of cod would not only render Skagerrak more robust to unpredictable catastrophic events but would also secure higher catches of cod and other fishes too.

The species composition in Skagerrak food web was parameterized with the data provided by our collaborators at the Flødevigen Research Station in Norway. The research of the PhD was supervised by Prof. Anna Kuparinen and Prof. Jeffrey A. Hutchings.

M.Sc. Sara Hočevar defends her doctoral dissertation “In the light of life histories, bet-hedging, and fisheries-induced change: case Atlantic cod (Gadus morhua)” on the 2nd of June, 2023 at noon 12:00 in lecture hall YAA303. The opponent is Professor Per Lundberg (Lund University, Sweden) and Custos Professor Anna Kuparinen (Ģ��ֱ��, Finland). The doctoral dissertation is held in English.

Publication details

The dissertation “In the light of life histories, bet-hedging, and fisheries-induced change : case Atlantic cod (Gadus morhua)” is available in the JYX repository: