Fisheries oceanography of northern pelagic fish species

People are familiar with marine fish species and the great variety of different species that are available in the market, such as herring, cod and sole. What may not be well known is that every individual fish goes through a long, risky journey during its life before reaching maturity. Most of the marine fish species are important prey for larger predators, such as larger fish in the ocean, marine mammals, birds and humans. Every individual female fish can produce many thousands to millions of eggs every year during the spawning season. The eggs (usually less than 2 mm in diameter) that live free in the environment, and depending on species, eggs either drift/float in the water or are attached to the bottom close to each other. The larvae that hatch from the eggs grow in size continuously until they reach a certain size and become adults. The first few weeks of its life are the riskiest and most of the eggs and larvae become prey
for other organisms living in the sea or perish due to unfavorable conditions, for
example high or low temperature or salinity. Those that survive to adulthood will participate with the rest of the population to the reproductive process, called spawning which takes place every year. The period of time that spawning takes place depends on the environmental conditions that each species has adapted to during the life of the species.
Here, I have found that temperature is a significant factor that strongly affects egg development and survival. There are large differences in the temperature range that maximizes survival among different species, but the sensitivity of egg development rate in a degree of temperature increase is similar among the 32 species and populations I analysed. I also found that adults spawn at temperature conditions that are generally close to the egg preferences, indicating that the egg stage is critical to the population abundance.
Two significant factors that regulate the time of spawning are the temperature at which the eggs and larvae can survive best and the availability of food for larvae. In general, at higher latitudes (northern North Atlantic) the optimal conditions are found during a narrower period and at lower latitudes (southern North Atlantic) the optimal conditions last longer. Temperature is also one of the factors that gives the signal to adults that the time to spawn has come. Another finding of my study is that species that have longer spawning seasons, at lower latitudes, are able to produce 10 times more eggs during their life time than species than have a limited spawning season. This may reflect the more un-predictable environmental conditions at lower latitudes. People are also familiar with the words "climate change" and "warming of the oceans". There is already evidence in the scientific community that the temperature in the surface water (0-75m depth) of the North Atlantic has been increasing by 0.11 oC
per decade since the 1970s. This will lead to a significant cumulative increase in the next few decades, with many consequences for marine organisms. One of the impacts will be the time that species start to spawn, and there is already evidence for earlier spawning in some North Sea fish species. A change like that may likely have a chain reaction, affecting larval stages and whether they will live in environments with high food availability. Warming temperature may also result in changes in the geographical distributions of species. If the nvironment becomes too warm at the areas that species live now, they may move to northern or deeper waters. This will leave space for other species, now living southern to move north. As a result the composition of the communities in the oceans will probably change. In my thesis, I also evaluated how expected climate change could affect the timing, location and success of spawning by herring in the North Sea. This species is an important species for the food-web and has an important commercial value for countries bordering the North Sea. I first developed a model that could describe the most important environmental conditions that determine herring spawning areas and times in the North Sea, and then used these model with future temperatures estimated by climate change models to estimate where and when spawning might occur in
future. I found that egg survival will generally remain high but that spawning times and locations will likely change. These changes could affect herring ecology (e. g., survival rates), if the larvae experience substantially different levels of food or predators than at present, and subsequently the North Sea herring populations. However, there can be differences in the sensitivity of some species to temperature changes, which for some will be greater compared to others. From the global perspective, humans will likely see the effects of climate change in the oceans by reduced availability of the species they consume now and increased availability of new species. In addition, there will likely be economic impacts on the local fishing communities. How species respond to climate change is a field of research that receives great attention because the responses will affect the management of fisheries