Getting to grips with evolutionary fisheries science

The IIASA Evolution and Ecology Program has played an internationally leading role in bringing attention to the evolutionary consequences of exploiting the world’s fish stocks. Fishing not only affects the numbers of fish, but also their heritable characteristics. While most examples of such evolutionary changes concern shifts in how long organisms wait before starting to reproduce, a new study on Atlantic cod shows that evolutionary responses to fishing also affect other aspects of reproductive behavior.

Fishing is an important source of food and nutrition for many people around the globe, especially in the developing world. Naturally, fish see only the other side of that coin—elevated mortality. It is widely acknowledged that even sustainable fishing practices inevitably reduce the abundance of fish populations. Until recently, an appreciation that fishing, when sustained over many generations, also affects the inherited characteristics of fish, was largely absent and is still developing. The Evolution and Ecology Program at IIASA has been assembling empirical evidence and developing the theoretical understanding required for finding the best ways to minimize unwanted evolutionary changes.

A very effective adaptation of fish to high fishing pressures is earlier maturation and the bulk of empirical evidence on the evolutionary effects of fishing confirms this. However, other traits are also likely to be important and the predominance of studies on maturation may in fact have more to do with the scarcity of data on evolutionary responses in other traits [1].

In the waters around Newfoundland in eastern Canada, fishing Atlantic cod has been the lifeline of local communities for centuries. The Department of Fisheries and Oceans (DFO) Canada has been conducting regular surveys on cod stocks since the mid-1980s, providing data to understand how cod populations have been developing. These data, on which the Evolution and Ecology Program have now collaboratively built with DFO scientists, allow researchers to study changes in the life-history characteristics of these stocks.

The cod stocks off Newfoundland are infamous for the collapse that led to the closing of cod fisheries in 1992, with full recovery from this disaster still pending. During the period leading up to the collapse, the size at which cod matured was gradually declining. Previous studies have shown that this decline was likely an evolutionary response to continued heavy fishing, as well as an indication of dangerously high fishing pressure, which could have served as a warning signal of the forthcoming collapse, had these analyses been available then [2]. Similar changes have been documented in numerous other cod stocks [1].

According to evolutionary theory, it is often advantageous for fish to invest more into their current round of reproduction if their chances of surviving to spawn again later are slim. This means that one might expect to observe larger reproductive organs (gonads) in the affected species. Fortunately, the DFO scientists measured the weights of gonads across large numbers of specimens, which offered a rare opportunity to test this theory [3].

The researchers found that male cod met their prediction: their relative gonad weights were increasing during the period when fishing pressure was high. Moreover, these trends levelled off, or even reversed after fishing moratoria reduced fishing pressure, which is the predicted pattern if fishing was indeed the driving force behind these changes. However, an analogous pattern is not evident in female cod: their gonad weights were stable or even slightly declining, despite the theoretical predictions. The researchers found this even more surprising, given that gonad weight is commonly considered more important for the reproductive success of female cod than of male cod [3].

Trends in relative gonad size of females (left column) and males (right column) for three stocks of Atlantic cod off Newfoundland (labeled 3L, 3NO, and 3Ps). In each panel, the blue curves show the estimated stock biomasses (right vertical axes). The two other curves show the model-predicted time trends of reproductive investments (measured by the gonadosomatic index GSI, that is, the ratio of gonad weight and gutted weight; left vertical axes) for the best models describing the data for the full time series (orange diamonds), or for the shorter period for which stock-biomass estimates are available (black crosses).

The reasons for the differences in the results for male and female cod are not yet properly understood. A possible explanation is the higher temporal variability in female gonad weights during the spawning seasons, making them a rather imprecise measure of the underlying energetic investments into reproduction. In addition, the results suggest that sperm production is more important for the reproductive success of male cod than previously appreciated.


[1] Heino M, Diaz Pauli B, & Dieckmann U (2015). Fisheries-induced evolution. Annual Review of Ecology, Evolution, and Systematics 46: 461-480.

[2] Olsen EM, Heino M, Lilly GR, Morgan MJ, Brattey J, Ernande B, & Dieckmann U (2004). Maturation trends indicative of rapid evolution preceded the collapse of northern cod. Nature 428 (6986): 932-935.

[3] Baulier L, Morgan MJ, Lilly GR, Dieckmann U, & Heino M (2017). Reproductive investment in Atlantic cod populations off Newfoundland: Contrasting trends between males and females. FACETS 2 (2): 660-681.


  • Institute of Marine Research, Norway
  • Department of Biology, University of Bergen, Norway
  • Hjort Centre for Marine Ecosystem Dynamics, University of Bergen, Norway
  • Fisheries and Oceans Canada, Northwest Atlantic Fisheries Centre, Canada

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