Knowledge in stock structure is vital for a proper fisheries management

Last changed: 25 July 2016

How to define a fish stock? The question seems so fundamental that science should have provided an answer already. From a genetic perspective the answer is rather straightforward: a stock/ population is comprised by the individuals that are taking part in reproduction. Because most fish during their short live span are unable to breed with so many other fish, the population concept has to be regarded over several generations: the fish that are a part of a population share a common gene pool.

So far is the definition rather unequivocal, but the main question is how is it that some individuals constitute a coherent stock? What are the mechanisms that keep one population together and delimit itself from other populations of the same species?

This issue has kept marine biologists busy for more than a century. Apart from lakes where there is more easy to comprehend how the obvious physical limits will define the stock (although there are, of course, many exceptions from such a simplification), the sea is an open system there both adult fish and their progeny can move around freely. This is especially true for our most important commercial fish species, as their eggs and larvae have a so-called pelagic phase – they are free-floating in the water mass and can be carried far away with sea currents from their original (natal) spawning grounds.

The integrity of fish stocks are therefore regarded as uncertain, simple because it very difficult to study the connection between where the parental fish spawned and the choice of spawning ground of their offspring. This relationship has been interpreted in two ways:

  • The hydrographic conditions will determine where the fish will settle and start to grow, and ultimately into which stock it will adhere. The fish might show spawning site fidelity (return to a specific spawning site), but the choice might not necessarily coincide with the natal spawning ground.
  • Fish have the ability to find its way back to their natal spawning ground, regardless of having being transported as very young to an unpremeditated nursery area.

The issue might seem a bit academic but has a bearing on how changes in stock abundance of cod should be interpreted. In the Öresund, Kattegat and archipelagos of Bohuslän, the status of the local cod stocks is crucial for the abundance of cod. Early life stages (eggs and larvae) are brought to the Swedish west coast by sea currents from spawning areas in the North Sea. Some years, this inflow can be high and give rise to strong year classes in the Skagerrak and northern part of the Kattegat. In other words, in the same nursery area there could be cod originating from different spawning stocks.

The phenomenon with inflow of recruits from the North Sea is especially striking along the coast of Bohuslän where adult cod today are missing. But even if many juvenile fish feed and grow in “cod empty” areas, the abundance of adults will not increase. For instance, the year class of 2003 was conspicuously high along the coast of Bohuslän. This year class could be followed in the monitoring fishing the year after in 2004. But the juvenile cod disappeared from the coast in 2005, just as the abundance of two year old cod increased off the coast. In short, the Swedish west coast might nowadays solely function as a nursery area for fish coming from the North Sea, but return as adults to their natal spawning grounds.

Cod should hence show a similar kind of homing behaviour as has been shown in salmonids. This theory was tested by tagging cod with DSTs (Data Storage Tags). The tags estimate and store clocked observations on temperature, depth and light intensity. The stored information gives the opportunity to get retrospectively estimate on positions. The tagging experiment confirmed that cod on the Swedish Skagerrak coast migrate to western part of the Skagerrak / North Sea at an age of 2-3 years, predominately during the spawning period. 

Litterature suggestions

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Knutsen, H. André, C. Jorde, P.E., Skogen, M.D., Thuróczy, E. och Stenseth, N.C. 2004. Transport of North Sea cod larvae into the Skagerrak coastal populations. Proceeding of the Royal Society ser B. 271: 1337-1344.

Lilly, G.R., Wieland, K., Rothschild, B., Sundby, S., Drinkwater, K.,Brander, K., Ottersen, G., Budgell, P., Carscadden, J., Stenson, G., Chouinard, G., Swain, D., Daan, N., Enberg, K., Hammill, M., Aqqalu, R.-A., Svedäng, H., & Vázquez, A. 2008. Decline and recovery of Atlantic cod (Gadus morhua) stocks throughout the North Atlantic. Resiliency of Gadid Stocks to Fishing and Climate Change pp. 39-66. Alaska Sea Grant College Program, AK-Sg-08-01, 2008.

Ruzzante, D.E., Mariani, S., Bekkevold, D., André, C., Mosegaard, H., Clausen, L.A.W., Dahlgren, T.G., Hutchinson, W.F., Hatfield, E.M.C., Torstensen, E., Brigham, J., Simmonds, E.J., Laikre, L., Larsson, L.C., Stet, R.J.M., Ryman, N. och Carvalho, G.R. 2006. Biocomplexity in a highly migratory pelagic marine fish, Atlantic herring. Proceeding of the Royal Society B 273: 1459–1464.

Secor, D.H. 2005. Fish migration and the unit stock: three formative debates. A review of ecological and historical issues related to stock connectivity and metapopulations. I: Cadrin, S.X., Friedland, K.D., Waldman, J.R. (eds) Stock Identification methods. Applications in fishery science. Elsevier Academic Press, MA s. 17-44.

Smedbol, R.K., McPherson, A., Hansen, M.M. & Kenchington, E. 2002. Myths and moderation in marine “metapopulations”? Fish and Fisheries 3: 20-35.

Svedäng, H. 2006. Torsken i Västerhavet. Publicerad i Kustfiske och fiskevård. Benny Lindgren & Håkan Carlstrand (eds.). Sportfiskarna. Bokförlaget Settern. s. 31-34.

Svedäng, H. 2007. Torskfiskar - Gadidae. S. Människan och faunan. Etnobiologi i Sverige 3. s. 431-434.

Svedäng, H. & Svenson, A. 2006. Cod (Gadus morhua L.) populations as behavioural units: inference from time series on juvenile cod abundance in the Skagerrak. Journal of Fish Biology 69 (Supplement C): 151–164.

Svedäng, H., Righton, D. & Jonsson, P. 2007a. Migratory behaviour of Atlantic cod Gadus morhua: natal homing is the prime stock-separating mechanism. Marine Ecology Progressive Series 345: 1-12. Utsedd som s k feature article.

Svedäng, H., Righton, D. & Jonsson, P. 2007. Defining ‘natal homing’ in marine fish populations; need for inference in fishery science: reply to Bradbury & Laurel (2007). Marine Ecology Progress Series 347: 309-310.

Vitale, F., Börjesson, P., Svedäng, H. & Casini, M. 2008. The spatial distribution of cod (Gadus morhua L.) spawning grounds in the Kattegat, eastern North Sea. Fisheries Research 90: 36-44.

Svedäng, H., Stål, J., Sterner, T. & Cardinale, M. 2010. Subpopulation structure in cod (Gadus morhua) puts strain on the management toolbox. Reviews in Fisheries Science 18: 139–150.


Henrik Svedäng, associate professor, researcher
Department of Aquatic Resources, Institute of Marine Research, SLU, +46 (0)10-478 40 63, 0727-16 90 72

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