Knowledge in stock structure is vital for a proper fisheries management
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.
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Earstones (otoliths) show that cod in the Sound is homing
In the inner part of fishes’ ears, earstones or otoliths are located. They constitute an integral part of the hearing and balance organs of fishes.
Otoliths are built from calcium-carbonate that precipitate on a matrix of proteins. These crystals are unique in the sense that continue to grow through life and are at the same time metabolically inert, i.e. they never transform. It means that if one cuts through the centre of the otolith, a time axis is obtained from the core to the edge of the otolith. In the otolith, also trace elements and isotopes are stored.
These elements vary in contents depending both on the animals physiology and on the content in the surrounding water masses. Taken together, these factors means that otoliths are truly “black boxes” from which, depending on our ability, can learn a lot things about the life history of individual fish.
One important question to ask is whether fish return from where they were born. For eel and salmonids, this seems to be the case: eel do not reproduce in lakes or coastal waters but swim back to the Sargasso Sea. Salmon may wander over the North Atlantic ocean but still find its way back to its own spawning site in its home river. Among marine fishes such as herring and cod, the opinions depart.
Some scientists believe that the passive dispersal of eggs and larvae by sea currents will determine to which spawning stock the fish will eventually adhere. On the contrary, other scientists emphasize the ability of fishes to migrate back to the natal spawning grounds constitute spawning stocks as behavioural entities.
The nature of the stock separation process is of very great importance for a successful fisheries management.
The Sound is a sea area which has attracted some attention in recent years, as the abundance of cod, especially big cod, is still very good. In the adherent Kattegat the cod stocks have been severely depleted since the 1970s. These marked differences have been related to the trawling ban in the Sound but also to a possible separation between the cod stocks in the Sound and Kattegat.
Genetical and behavioural studies were conducted in order to study how fish stocks differentiate between different localities. Behaviour is studied by tagging (marking) the fish. Our tagging studies indicated a separation between the cod stocks in Sound and Kattegat, whereas the genetical studies were not able to differentiate between the two stock units.
As a contrast to these studies, we investigated the content of trace elements in the otoliths from in tagged and recaptured fish. It was shown that three different spawning sites were used by the tagged fish: in the southeastern part of the Kattegat, in the Sound and in off the coast at Kullen between the northern Sound and Kattegat. At Kullen, no fish had been tagged but all recaptures were fish that had swam from the south in the Sound or from the north in the Kattegat.
Analyses of trace element contents (for instance strontium, iron and manganese) were made at the Institute of Nuclear Physics at Lund university. It was the content of trace elements in the very core of the otolith that interested us, as it is this part of the otolith that reflects the chemical composition of the surrounding water masses just after fertilisation.
The results were striking: all three spawning areas deviated from each other whereas the otoliths tended to be similar within the same spawning area. This shows clearly that fish tend to return to their natal spawning area, where they once were produced. It means that we have to cautious about every spawning aggregation as they represent the production units of the marine fish stocks.
It also implies that if spawning aggregations disappear, they will not so easily be re-established due to fact that juvenile fish tend to return to their parental spawning grounds.
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Svedäng, H., André, C., Jonsson, P., Elfman, M. & Limburg, K. 2010. Homing behaviour and otolith chemistry suggest fine-scale sub-population structure within a genetically homogenous Atlantic cod population. Environmental Biology of Fishes 89: 383–397.
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