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Growth and Life History

Using zebra fish to model populations

Within MAR-ECO, Professor Mikko Heino undertook studies to extract life history information about fish from their length distribution data. Often, he says, this is the only information that we have about fish stocks.

Recently Heino, who holds a joint position at the Norwegian Institute of Marine Research and the Department of Biology at the University of Bergen, was one of four young researchers who was awarded funding from this year’s Bergen Research Foundation’s recruiting programme.

Mikko works in fishery science and will use the funding to establish a research group that will, among other things, establish a research model for studying life history evolution in commercially exploited fish populations.

Fish have a variety of life history strategies. Like many cold-blooded organisms, they generally have an ‘indeterminate’ or continuous pattern of growth unlike that of warm-blooded organisms such as human beings, which have a ‘determinate’ pattern that stops at reproductive maturity.

The problem is not that simple, explains Heino, who cites herring and mackerel or halibut as examples of fish with very different growth curves.

After sexual maturity, herring continue to grow very slowly, while there are always reports of catches of especially large mackerel or halibut, which continue to grow unabated.

Natural selection vs. fishing

What forces drive life history patterns and growth in particular? For thousands of years, fish populations have been affected by natural selection due to factors in their environment. For the last several hundred years the effects of these ‘natural’ conditions have been overshadowed by human influences; namely fishing. Instead of natural selection it is harvesting factors that have the greatest effects on fish evolution.

How can the evolutionary effects of harvesting be quantified? How can these effects on a population’s productivity or commercial viability be measured? How can fisheries be managed sustainably?

Heino explains that the Norwegian cod fishery makes an interesting case study because it is historically one of the most important Norwegian fisheries and thus has some of the longest, most extensive data sets.

North East Atlantic cod has a life history strategy that involves returning to the coastal waters of northern Norway to spawn every spring. Historically, the cod fishery was mostly targeting spawning cod along the coast where the cod were aggregating to spawn and were easily accessible even with small boats.

One hundred years ago, this population had an average age of 9-10 years for attaining sexual maturity. This was probably an evolutionary adaptation as the feeding grounds in the Barents Sea were largely inaccessible to fisheries and the cod could feed there in relative safety.

The advent of technologically advanced fishing vessels has, however, changed all this, and since the WWII fishing pressure in the Barents Sea has been high. It is no longer evolutionarily advantageous to delay maturation, and the fish are investing in reproduction earlier and are not growing as large.  In addition the spawning grounds have contracted and now are limited to waters around Lofoten.

Crash of a fishery

The Norwegian Institute of Marine Research is very concerned with understanding fish stock dynamics in order to be able to advise government agencies on how to manage these important natural resources. The crash of the Norwegian spring herring in the 60’s and the crash of the Canadian cod fishery are lessons no one wishes to repeat.

The herring population has recovered and is an economically viable fishery today, and, if fact, this year’s quotas are the highest since the fishery’s collapse. However, the mechanisms of the evolutionary recovery are not fully understood. Heino’s group is going to develop a model population to study the effects of selective harvesting on population dynamics.

Modeling a population in the lab

Studying marine populations is difficult. Establishing a model population in the controlled setting of a research laboratory would enable scientists to test a number of assumptions they believe might affect wild populations.

Zebra fish and guppies are considered ideal lab models. Much is known about their biology. They have been successfully reared under laboratory conditions for many years. In fact, points out Heino, they may be too successful as lab models; too domesticated and not sufficiently “wild”! The Department of Biology at the University of Bergen actually has a small population of “wild” zebra fish from India, but researchers have not as yet been successful in breeding them.

Once Heino and his research group have established lab populations, they will conduct selective harvesting experiments to observe their effects on the population dynamics and productivity. They will also study effects on other life history characteristics such as egg size, growth patterns, behaviour etc.

Relevance to deep-sea populations

Heino’s work may be very relevant to newly developing deep-sea fisheries. These fisheries have been exploited for a relatively short period, explains Heino. In many ways they are similar to the coastal fisheries of several hundred years ago. Already data from projects such as MAR-ECO has shown that the length distributions of many deep-sea species are much different from that encountered in coastal species. Are deep-sea fish just different or do these differences reflect that these populations are as yet relatively unaffected by fishery pressures?

The results of Heino’s new project may be of great relevance to the sustainable development of both deep-sea and coastal fisheries.

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