Age at first reproduction has declined markedly in cod the last decades (Figure 4.6.6). This may have considerable consequences for cod recruitment and the role of cod as a top predator in the ecosystem. In the 1940s, a cod typically reproduced for the first time when it was between 9 or 10 years old. In the 1990s, average age at first reproduction had declined to between 6 and 7 years.
Re-reading of old cod-otholits suggests that age had been over estimated in the beginning of the time series in the above figure, and that the decline in age at maturity therefore has been less pronounced than suggested here (Zuykova et al 2009).
The possible explanation for the phenomenon is that declining age at maturation in Northeast Arctic cod is an adaptive response to high fishing pressure through many years and thus involves genetic changes in the population (e.g. Law and Grey, 1989). The following illustrates the mechanisms involved. Because the number of offspring that a cod can produce increases considerably with body size, older fish generally produce more offspring than young fish. Before 1930, fisheries were almost exclusively on the coastal spawning grounds and fishing mortality was considerable here. In the nursery areas in the Barents Sea, fishing mortality was negligible. In this situation, the cod that produced the most offspring through their life time were those that stayed for a long time on the nursery grounds before entering the spawning grounds, as they enjoyed the advantage of growing large in an area protected from fishing mortality. With the introduction of industrial fisheries in the nursery areas in the Barents Sea from the 1930s and onwards, the rules of this game have been turned upside down. The high fishing mortality in these areas today means that there is very little chance that a cod will survive to the age of 10 years. Those that get to reproduce, and make up the basis for the stock in the future, are those that spawn early. The “maturation at old age genes” are slowly removed from the stock.
Reduced age at maturity may affect the reproductive capacity of the cod stock and the cod’s role as an important top predator in the ecosystem. Because eggs spawned by older cod are more viable than those from younger cod the reproduction potential of the stock has been negatively affected by the development (see Sundby 2000 for references). In addition, the decline in average age at maturity has caused the spawning stock to be made up of fewer age groups. This has made recruitment more dependent on environmental factors in recent decades compared to previous times when more age groups of older fish participated in the spawning (Ottersen et al. 2006). Note that these effects will happen also because fishing reduces the age structure in the population, but any evolutionary effects may exacerbate such an effect and make it last longer if fishing pressure were reduced.
Fishing is also expected to lead to larger gonads for fish of a given size, or higher reproductive investment in general (Dunlop et al. in press, Enberg et al. 2009; changes reported for North Sea cod in Yoneda and Wright 2004). Over time, such evolution of maturation age and reproductive investment may lead to a larger proportion of the total biomass becoming sexually mature (Enberg et al. 2009). A consequence is that the stock may become more resilient to fishing, and stocks that have not undergone such life history evolution might be more prone to collapse under high harvest rates (Enberg et al., 2009).
If the adult cod generally becomes smaller because of maturing earlier, its role as top predator may change because smaller cod might eat a different composition of prey species than large cod. This might change the way cod affects its prey species, and have significant overall effects on the ecosystem.
In addition, the changes in body size that follows with evolutionary changes in age at maturity may affect spawning migrations. In a theoretical model the observed change towards maturation at earlier age and smaller size is likely to also shorten the southward spawning migration, such that the cod will spawn on more northerly locations (Jørgensen et al. 2008). The reason for this is that even though spawning in southern locations seems to be beneficial for the larvae because they will spend a longer time in warmer water as they drift towards the Barents Sea (Opdal et al. 2008), the southwards migration against the current is energetically costly, and smaller spawners do not have the energy reserves required for it. This might have consequences for the geographical allocation of fishing effort.
Models suggest that fishery induced changes in age at maturity may be very slow to reverse (Law and Grey 1989). Thus, parts of the changes may already be very hard to reverse, and this may become even more difficult as the current fishing practice continues.Different types of fishing gear remove different individuals. For example, gillnets select fish of a certain girth, whereas small fish may slip through the mesh and large fish may not get caught. This is different from a trawl, where sorting grids allow small fish to escape but most larger fish are caught. A model for Northeast Arctic cod suggests that trawling may lead to evolution towards early maturation even at low fishing intensities, whereas fishing with gillnets can take place at moderate rates without such evolution to occur (Jørgensen et al. 2009). Hutchings (2009) reached similar conclusions. These models used only gear selection based on body length, and although certain gear may also select hungry fish (Philipp et al. 2009) or fish of a different girth, but evolutionary effects of such harvesting has not yet been investigated theoretically. Gillnets that allow old and large fish to escape can also be beneficial for recruitment to the population if maternal effects make offspring from older mothers more viable (Law 2007, Venturelli et al. 2009).