Oliver Jones explains how to use knowledge of individual behaviour and an ageing technique to save entire populations of ever decreasing fish populations

"Fish populations in crisis", announces one headline. "Only 50 years left for sea fish" declares another. According to most fisheries' scientists, fish stocks around the world are in trouble. Despite bigger boats, better nets and new technology for spotting fish, the world's fishing fleets are actually catching less (the global catch fell by 13% between 1994 and 2003). In order to manage fish stocks in a more sustainable manner a better understanding of fish ecology is needed. But how can one obtain such an understanding? Curiously enough, some of the answers may be found by studying a small freshwater fish which does not even taste very nice.

At first glance, the European bitterling (Rhodeus sericeus) does not look like a particularly interesting fish. It is fairly small (they grow up to 10 centimetres long), it has no fancy colours (the exception being the males in mating season) and, despite being food for a variety of predators, such as perch (Perca fluviatilis) and pike (Esox lucius), it is not endangered. What is very interesting about bitterling, however, is their mating strategy.

Members of the bitterling subfamily are unique among fish species because they have an obligate spawning relationship with living freshwater mussels. During the breeding season males defend territories around mussels and perform courtship displays to attract females for spawning. Interestingly, like many other fish species, some males prefer to employ the alternative strategy of sneaking into the territory of another fish rather than going to the trouble of defending their own.

Mussels are filter feeders: they feed by drawing a current of water in through an inhalant siphon and out through an exhalent siphon, and filtering out food particles. During the mating season female bitterling develop long egg tubes, ovipositors, which may extend to the end of their tail. They use these to place their eggs into the gills of a mussel. Males fertilise the eggs by releasing sperm into the inhalant siphon, so that the water current generated by the mussel carries the sperm to the eggs. The young then develop inside the mussel for about a month, eventually leaving as actively swimming larvae. As an interesting aside, this behaviour was once used as a pregnancy test for humans, since the female bitterling's ovipositor would often extend upon exposure to the hormones in a pregnant woman's urine. However, it was not an especially reliable test, sometimes predicting men were pregnant, and quickly fell out of favour.

Four species of unionid freshwater mussel (Unio pictorum, Unio tumidus, Anodonta anatinea and Anodonta cygnea,) are commonly used as spawning hosts by bitterling (although females have been shown to prefer to spawn in the first three while avoiding A. cygnea where possible). The catch for all this free parenting is that the mussels release their own larvae, known as glochidia, into the water column. During their early development these attach themselves to adult bitterlings in the vicinity, before dropping off and settling in the substrate to metamorphose into young mussels sometime later. This relationship has previously been thought of as symbiotic, since each side gains something. Interestingly however, recent work indicates that bitterling may in fact be parasitic. This is because they go to elaborate lengths to avoid becoming hosts to mussel larvae whilst imposing an energetic cost on the mussels forced to act as foster parents to young bitterling.

Because bitterling are totally reliant on freshwater mussels for their survival they are a valuable model species in behavioural, population and evolutionary ecology studies, especially those dealing with the links between behaviour and population dynamics. This is because their spawning sites can be easily quantified, manipulated and assessed for quality, while the fish themselves are easily observed in both the natural environment and under laboratory conditions. Nevertheless, ascertaining population data is difficult without a reliable way of ageing individual fish. Luckily there is a reasonably easy way to do this, through what is known as otolith analysis (see text box).

Using otolith analysis it is possible to calculate and compare growth rates among fish from different populations and from different habitats within populations. One study carried out a few years ago applied this to bitterling populations in the Czech Republic, which is the centre of their natural range in Europe. What was curious was that the results of the study indicated that growth rates of fish of the same age from both high and low quality habitats were the same. This was despite the fact that individuals in low quality habitats were both considerably more vulnerable to predation and had less available food than those in high quality habitats.

This apparent anomaly can be explained by considering the type of competition operating within the system. There are two types of competition that are likely to operate in fish communities. Exploitative competition (also called scramble or resource competition) involves the removal of resources by one, which reduces the availability of resources for others but implies no direct interaction amongst individuals. Interference competition (also called contest competition) involves a direct behavioural interaction between competitors, which reduces the net energy gain of one or both interactors.

If interference competition is operating within a system, competitively superior individuals will occupy high quality habitats and competitively inferior individuals will be relegated to low quality habitats (referred to as despotic distribution). Conversely, if exploitative competition is in operation, ecological theory indicates that individuals will choose the habitat with the highest net profitability. As a consequence, individuals will be spread out among all available habitat types so that the fitness of all remains approximately equal.

Since the growth rates of juvenile bitterling in the study were determined to be equal in both habitat types (and assuming growth rate gives a useful indication of overall fitness) it is reasonable to assume that exploitative competition operates in this particular system, at least for fish of the same age group. Increased competition in the high quality habitat meant that there was less food available to the individuals present; hence the decision of some fish to feed in the lower quality areas is reasonable. Although there were fewer, or lower quality, food items present, the competition for resources was reduced in trade-off for the higher risk involved in utilising them.

Now, bitterling are not a particularly important fish for humans so you could be forgiven for wondering what all this has to do with fish stocks. Well, the simple answer is that the rules governing the population dynamics of bitterling are also likely to hold true for (or at least be similar to) those governing the population dynamics of other types of fish that are harder to study, for example open ocean species. The more we can understand about how fish behave and how that behaviour affects population dynamics and other aspects of their ecology, the more successful our management of fisheries and marine reserves is likely to become. With many fish species on the verge of extinction but more people than ever reliant on them as a food source, that can only be a good thing.

How to age a fish

Otoliths (literally ear stones) are a calcium carbonate mass located in the inner ear of many fish species. They supply the individual with information about changes in direction, position and speed, help them balance and are also capable of detecting sound. Otoliths grow via the continual deposition of calcium carbonate over a 24 hour period. At night, when the fish is inactive or asleep (and hence not feeding) this deposition slows down to a very slow rate and it is this that causes the dark rings. In contrast, faster calcium carbonate deposition during the day causes the wider white bands. Much like ageing a tree by counting the rings in its trunk, by counting the rings in an otolith one can calculate the age of the fish. This is an especially useful test when checking commercial catches to make sure fishermen are not catching immature fish. To do this, the otoliths must first be dissected out from the fish and examined under a light microscope, with a video camera attached. The camera allows the image of the otolith to be projected onto a screen to allow a more accurate ring count.

A short film on a related topic called Cinequarium is available online

Oliver Jones is a postdoc in the Department of Biochemistry