Other marine animals are also affectedNot only do fisheries alter the natural species structure of the fish that are being fished for; they also have an impact on the stocks of animals that are taken as bycatch. U.S. researchers have calculated that at least 200,000 loggerhead sea turtles and 50,000 leatherback turtles worldwide were caught incidentally in the year 2000 by tuna and swordfish fishers. The turtles are caught on the hooks of “longlines”. These are usually several kilometres long and can be fitted with thousands of baited hooks. If the turtles snap at these they will be hooked. Some are able to free themselves and others are thrown live back into the ocean by the fishermen. But thousands die an agonizing death. Tests are now being carried out to shape the hooks so that the turtles will no longer be caught by them. The longlines can also be fatal for albatrosses because they do not sink to the working depth immediately after being let out, rather they float for a while at the surface and attract the birds. Environmental organizations estimate that hundreds of thousands of sea birds are unintentionally killed annually worldwide by longline fishing. New methods are therefore also being tested by which longlines are deployed through tubes that extend up to 10 metres below the surface, so that albatrosses cannot see or reach the bait.
- 1.14 > This bluefin tuna, weighing 268 kilograms, fetched a price of 566,000 Euros at a fish auction in Tokyo in January 2012. It was bought by Kiyoshi Kimura (left), president of a sushi gastronomy chain. In early 2013, Kimura even paid a full 1.3 million Euros for a tuna. That translates into a price per kilogram of more than 6000 Euros.
- In the Baltic Sea, the harbour por-poise is also endangered as bycatch. There are only an estimated 500 to 600 individuals remaining in the eastern Baltic Sea. The harbour porpoise was hunted here for decades. Severe icy winters are also a strain on them. Today, every unintentionally caught animal brings the stock closer to extermination. It is a near tragedy that the eastern Baltic Sea harbour seals very rarely mate with their relatives in the North Sea and western Baltic Sea. The North Sea stock is comparatively large. Researchers estimate it to be around 250,000 animals. Because the eastern animals do not mate with their western relatives, it is feared that the species could die out in the Baltic Sea. This would mean a loss of species diversity in the region.
Genotype Genotype refers to the total genetic information of an organism that is stored in the cell nucleus of each body cell. Among individuals of a species most of the genes are identical. But their combination is unique for every individual.
Phenotype The phenotype is expressed in the appearance of an individual: the observable characteristics of the individual genotype. Phenotypic attributes include eye colour, psychological traits, or genetically caused illnesses.
The fisheries influence evolutionIntensive fishing, however, also changes the biological diversity in another way. Scientists are now discussing the phenomenon of fisheries-induced evolution. When the fisheries primarily catch large and older individuals, then, over time, smaller fish that produce offspring at an earlier age become more successful. The fisheries thus critically upset the natural situation. In natural habitats that are not affected by fisheries, larger fish that reach sexual maturity at a greater age are more dominant. Their eggs have lower mortality rates. The eggs and larvae can better survive phases of hunger in the beginning because they possess more reserve substance, more yolk, than the eggs and larvae of parents that reproduce at a younger age. The entire stock benefits from this because many offspring are regularly produced, which preserves the stock. Under heavy fishing pressure, on the other hand, the animals that primarily reproduce are those that are sexually mature at a smaller size. But they produce fewer eggs, and their eggs have higher mortality rates. Through computer models and analyses of real catch data, and using the example of the northeast Arctic cod, researchers have been able to show that this fish stock has actually undergone genetic alteration through time. Fish with the genotypic trait of becoming sexually mature at a young age and small body size have become more successful. This is true for both males and females. To illustrate this, the research-ers have employed catch data in their model that extend back to 1930 and document the gradual changes with respect to age, size, and reproductive capacity. The study was based on especially detailed data sets of the catches in Norwegian waters. Originally the northeast Arctic cod became sexually mature at an age of 9 to 10 years.
- 1.15 > Over decades of fishery, plaice in the North Sea that achieve sexual maturity with a smaller body size have grad-ually become predominant. This relationship can be clearly depicted by using different probabilities (p) in mathematical models. The body length (L) of 4-year-old plaice that will become sexually mature in the coming season with a 90 per cent probability (p90) is illustrated. As the graph shows, this body length (Lp90) has decreased significantly over recent years.
- In the northeast Atlantic today, the cod is sexually mature at 6 to 7 years old. It is notable that this fisheries-induced evolution has occurred over a period of just a few decades. Experts feel that one reason for this is that the fisheries exert a much greater pressure than natural selection factors such as predators or extreme environmental conditions, such as heat or cold. The computer models also indicate that it would take centuries for the effects of the fisheries-induced evolution to turn around – even if the fisheries were completely stopped. In actual practice, the effects may even be irreversible. Within the past 10 years fisheries-induced evolution has been verified for a number of species, including the North Sea plaice. The effect of fisheries is thus exactly the opposite of what an animal breeder usually aims for. The animal breeder, as a rule, selects the largest and most productive animals in order to continue breeding with them. As a result of the fisheries, by contrast, precisely those older and larger animals with the highest reproductive capacity are killed. >