Climate Impacts: Field of Action Fishery

In the maritime sector, the impacts of climate change on fish stocks (e. g. distribution, yield) must always be considered and assessed against the background of other stress factors such as habitat loss, marine pollution and fisheries.

Stress caused by increased ocean temperatures: As cold-blooded animals, fish are strongly influenced by the surrounding water temperature. For many fish species, a water temperature deviating from their specific optimum temperature means stress. Exposure to this stress over a longer period of time affects the metabolism and thus the growth, reproduction and mortality of the fish. Thus, increased water temperatures cause faster growth, faster egg development and increased metabolic rates. For Germany, changes in water temperature in the North Sea and Baltic Sea are relevant. In the period from 1969 to 2017, the mean annual surface temperature of the North Sea warmed by 1.3 degrees. The absolute warming of the Baltic Sea in the period from 1980 to 2015 is 1.6 degrees at the surface and 1.9 degrees at a depth of 20 metres for the western Baltic Sea. Depending on the climate scenario, it is assumed that by the end of the century the Baltic Sea will be 2 to 3 degrees warmer on average at the surface and that summer surface water temperatures above 18 degrees Celsius could occur up to one month longer than before.

Effects on fish phenology: Increased sea temperature can change the phenology of fish in such a way that the temporal synchronisation of certain developmental phases dissolves. For example, the water temperature in spring determines when the spawning migration of Atlantic herring (Clupea harengus) into the Baltic Sea begins and female herring lay their eggs. If these water temperatures are reached earlier in the year, the fish eggs develop faster and the larvae hatch sooner. Studies have shown that the spawning migration and the "start signal" for spawning have moved further and further forward in the calendar in past years. This can lead to the herring larvae hatching before the spring plankton bloom has begun and thus there would be hardly any plankton food available to them, i. e. the food relationship is decoupled. The lack of food supply leads to increased stress on the juveniles in their most sensitive phase of life and to death. Such an asynchronous development ("food mismatch") is, along with intensive fishing, one of the explanations for the observed decline of herring stocks in the Baltic Sea since the 1990s.

Stress due to oxygen deficiency and eutrophication: Oxygen availability for marine wildlife is directly related to water temperature. As the temperature rises, gas solubility decreases, but in contrast, the demand for oxygen for metabolism increases. The number of near-shore waters with oxygen deficiency and so-called "dead zones" is expanding, affecting coastal ecosystems and the fishing industry. Nutrient input from the land is one of the main causes of the "dead zones" in coastal waters. The high nutrient load (eutrophication) of coastal waters benefits filamentous brown algae (e. g. of the genera Pilayella and Ectocarpus), which can spread en masse. These algae grow on the seagrasses and spawning herbs that form the main herring spawning beds. This causes long-term damage to the plant beds. The brown algae also benefit from mild winter temperatures.

Stress due to ocean acidification: If the carbon dioxide content in the atmosphere increases, the oceans absorb more carbon dioxide (CO2). As a result, the pH value of the seawater decreases and the oceans are becoming acidic. Since the beginning of the industrial revolution, the oceans have become almost 30 per cent more acidic, with the average pH of the sea surface dropping from 8.2 to 8.1. By 2100, the pH of the oceans is expected to decrease by another 0.3 to 0.4 units, making seawater 100 to 150 per cent more acidic. Fish are considered relatively insensitive to acidification. Nevertheless, it could have a direct impact on fish behaviour and physiology. If the pH of seawater drops, the pH in the body fluids of most creatures also drops and an acid imbalance can occur. Fish can regulate their acid balance within hours or days. However, this costs energy that may be lacking for growth and reproduction. Acidification of the oceans poses a threat to calcifying organisms (e. g. shell-forming plankton). Acidic water impedes the formation of inner skeletons or protective shells of calcium carbonate (lime), and the shells and calcareous skeletons of these marine organisms become thinner or possibly dissolve. Since these organisms are a basis of the food pyramid in the sea, there are far-reaching consequences for the food chain in the sea with consequences also for the fish populations.

Changes in the species composition and spatial distribution of fish: The change in ocean temperature could result in values that lie outside the ecological preference of fish species. This would result in changes in fish species composition and distribution, as can already be observed in the North Sea and the Baltic Sea. As an open marginal sea of the Atlantic, the North Sea generally offers fish species more opportunities to shift their habitats with the climate change-induced rising sea water temperatures. Thus, it can be observed that the habitat of cold-loving fish species, e. g. Atlantic cod (Gadus morhua), Atlantic mackerel (Scomber scombrus), Capelin (Mallotus villosus), Saithe (Pollachius virens) and Blue whiting (Micromesistius poutassou) is shifting towards the pole and thus to cooler regions. The "nursery room" Wadden Sea has also become too warm for plaice (Pleuronectes platessa). The juvenile stages are already migrating to the North Sea. On the other hand, the rise in water temperatures and the absence of very cold winters enable warmth-loving fish species from more southerly marine areas to immigrate, overwinter and reproduce in the North Sea. These include, for example, the hake or pike (Merluccius merluccius), the sardine (Sardina pilchardus), the anchovy (Engraulis encrasicolus), the red mullet (Mullus barbatus) and the sea bass (Dicentrarchus labrax). In the Baltic Sea, the cold-loving cod, a key source of income for many fisheries in the western Baltic, may migrate north towards the Arctic. In contrast, other fish species may migrate into the Baltic Sea and become native. The thick-lipped mullet (Chelon labrosus) now regularly moves in from the North Sea in spring when the water settles at 11 degrees, stays until autumn and then migrates back to the North Sea to spawn. Anchovy, sardine, red mullet and dorade are also migrating more and more into the Baltic Sea around Jutland (Denmark). In the Baltic Sea, the blackmouth goby (Neogobius melanostomus) has also been spreading in huge numbers since the last few decades. Its natural distribution areas are the coastal areas of the Black and Caspian Seas. The goby is tolerant of temperature fluctuations, low oxygen content and varying salinity. Due to its proliferation and reproductive potential, it is considered an invasive species that competes for food and space with many native species. The Federal Agency for Nature Conservation in Germany included the blackmouth goby in the black list of invasive species as early as 2010.

Indicator from the Monitoring on the DAS: Distribution of thermophilic marine species

In the case of inland waters, a distinction must be made between the effects of climate change on fish stocks in running waters (rivers and streams) and standing waters (lakes). Above all, rising temperatures and increasing low-water phases as well as changes in the composition and spatial distribution of fish species have an impact on fish stocks and thus on inland fisheries. In inland lakes, the stratification of surface and deep water is also affected.

Stress due to increased water temperatures: Higher water temperatures can also lead to increased stress in fish in flowing and standing waters. This particularly affects the time of reproduction and the early stage of a fish's life as an embryo. Even a deviation of + 0.5 degrees Celsius from the upper temperature limit (21.6 degrees Celsius in summer) can lead to a negative impact on brown trout populations, which tend to prefer cool and oxygen-rich running waters. High water temperatures also affect the respiration of the fish. Warming reduces the gas solubility of the water, which lowers the oxygen concentration in the water and results in a higher risk of fish mortality due to oxygen deficiency. In inland lakes, fish species that tend to be cold-loving and require oxygen, such as whitefish (Coregonus species) and burbot (Lota lota), have their population dynamics affected by rising water temperatures. For example, the burbot in Lake Constance in South Germany seems to have stopped reproducing several years ago. In the egg stage, it is particularly sensitive to temperature. Temperatures as low as six to seven degrees Celsius damage egg development in the early stages and can lead to total failure.

Changes in the thermal structure and oxygen ratios of inland lakes: One of the most obvious effects of climate warming on deep inland lakes concerns their thermal stratification. Deep lakes are characterised by different warm water layers in summer: warm water near the surface (epilimnion) is layered over the cold deep water (hypolimnion). In the cold season, this layering is lifted. Oxygen enters the deep water through circulation, and the lakes are mixed. Due to the consequences of climate change, the stratification period is prolonged. Stratification begins earlier in the year and becomes more stable. The vertical exchange that otherwise takes place between the layers decreases. During the stratification phase, only the surface water is supplied with atmospheric oxygen. Only the oxygen that is not consumed by degradation processes remains in the deep water. For Lake Constance, it has been shown that vertical winter mixing already occurs less frequently and more often remains weak or incomplete. This trend is expected to continue. Due to higher water temperatures and nutrient concentrations, larger biomasses are formed by algae in the surface water. After they die, these sink into the deep water and are decomposed there, consuming oxygen. Thus, the oxygen content of the deep water decreases and oxygen-free (anaerobic) zones form in the hypolimnion. As a result, nutrients (e. g. phosphorus) previously bound in the sediment can be released by chemical processes. This leads to "internal fertilisation" and thus to a degradation of water quality. Under warmer climate scenarios in the future, this means that external nutrient loads (e. g. from agriculture) to standing waters will have to be reduced even more than before to compensate for the effect of "internal fertilisation".

Algal blooms: Higher water temperatures, a longer stratification period and high nutrient concentrations in surface water favour the development of cyanobacteria (blue-green algae). Due to their often filamentous structure and tendency to form colonies, they largely evade feeding pressure from zooplankton. Cyanobacteria can thus form dense bloom carpets ("algal blooms") on the water surface. Most cyanobacteria are harmless to humans, but can severely limit the recreational value of a lake as a bathing water. However, some genus of cyanobacteria produce a number of secondary metabolites that have a toxic effect and can harm fish. Some of the toxins can also be hazardous to human health and cause allergic skin reactions and inflammations in bathers.

Stress due to drought and heavy precipitation: As a result of high water evaporation during heat waves and very low summer precipitation, critically low water levels can occur in small rivers. In extreme cases, the water bodies dry up, which can lead to catastrophic developments in fish stocks and the entire ecosystem. The reduced water flow can reduce the continuity and connectivity of the water habitats, especially in smaller river headwaters. For migratory fish species, this has negative effects on spawning. In shallow lakes, low water phases can lead to accelerated silting up of structured bank areas and thus to a loss of important reproduction and juvenile fish habitats. Low water levels can also increase predation pressure by fish-eating animals (e. g. cormorants) and thus lead to further damage to fish stocks. Commercial fishing is severely restricted in these marginal areas due to low water levels. Heavy rainfall events can lead to increased water erosion in running and standing waters and consequently to an increased input of fine sediments into the waters bodies. In running waters, this input leads to increased compaction or "clogging" of the gravel gap system of the river bed and thus to a loss of habitats and spawning sites (e. g. for gravel-spawning trout species). In inland lakes, increased inputs of organic substances and nutrients can promote algal development and bacterial activity and lead to oxygen deficiency.

Changes in the composition and spatial distribution of fish species: In running waters, higher water temperatures may lead to a decline in the distribution of cold-loving fish species of the salmonid or salmonid family, e. g. brown trout (Salmo trutta), rainbow trout (Oncorhynchus mykiss), brook trout (Salvelinus fontinalis), grayling (Thymallus thymallus) and coregone species (e. g. whitefish, vendace). This would be associated with a reduced fishery exploitability of these stocks. Other cold-loving fish species such as burbot (Lota lota), bullhead (Cottus gobio) and brook trout (Telestes souffia) also prefer summer-cool lakes or cool rivers as ideal habitats. It can be assumed that with increasing warming of the flowing waters, the habitats suitable for cold-loving fish species will shift to higher altitudes and deeper stretches of water in the long term. In other regions, where these alternative habitats do not exist, these species will drastically decline. On the other hand, warmth-tolerant fish species will benefit from increased water temperatures in running and standing waters and will be able to expand their habitats, e. g. carp (Cyprinus carpio), bitterling (Rhodeus amarus), roach (Rutilus rutilus), gudgeon (Gobio gobio), bleak (Alburnus alburnus), bream (Abramis brama) and catfish (Silurus glanis). For fish species in standing waters, long-distance immigration and emigration possibilities are more problematic due to the often isolated location of the water body. In most cases, they can only move between habitats within a water body. Fish species with a broad temperature tolerance, such as many cyprinids (carp species), will continue to spread in standing waters, while stocks of cold-loving fish species (e. g. vendace, whitefish), on the other hand, could decline in the long term or become regionally extinct.

Indicators from the monitoring on the DAS: Occurrence of thermophilic species in inland waters – case study, Herring larvae in the Greifswalder Bodden

Spreading of invasive fish species: Due to climatic changes, alien fish species can become established and spread in running waters. This includes, for example, the bluegill (Pseudorasbora parva), which originally comes from East Asia and is now widespread throughout Germany. It competes with domestic animals for food, displacing white fish species such as the Moderlieschen (Leucaspius delineatus) or the Bitterling. In winter, it feeds parasitically on the muscle flesh of carp and tench, sometimes inflicting deep wounds on them. Another representative of an invasive species is the common sunfish (Lepomis gibbosus), which is originally native to eastern North America. As a food and habitat competitor and spawn predator, it is a threat to the native fish population. With its listing on the European Union's list of invasive alien species of Union-wide importance in 2019, trade in the species is banned in the European Union.

Spread of diseases and parasites: Rising water temperatures will also affect the spread and frequency of fish diseases. Proliferative kidney disease in fish is caused by the parasitic cnidarian Tetracapsuloides bryosalmonae. It affects trout fish (salmonids), grayling and pike in wild stocks and aquaculture and can lead to drastic reductions in stocks. Another fish disease that can occur is fish furunculosis, which is caused by the bacterium Aeromonas salmonicida and particularly affects brown trout and char. It occurs in all age groups and is more frequent in summer when water temperatures are high, oxygen is scarce and other stress factors are present. The freshwater eel blight, caused by the bacterium Aeromonas hydrophila, occurs in shallow and nutrient-rich lakes with high stocking density at water temperatures above 24 °C for eels.

In Greifswalder Bodden, spring storms cause early herring larvae to hatch with scarce food.

Source: Dorothee Moll

In the southern and central North Sea, catches often include Lusitanian species like sardines.

Source: André LABETAA / stock.adobe.com

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In Germany, the effects of climate change on fish farming in aquacultures particularly affect trout and carp farming. Factors such as the increase in water temperatures, the reduction in oxygen content, the increase in extreme weather events (e.g. dry periods, heavy rainfall events) and the spread of pathogens and parasites play a significant role.

Stress due to increased water temperatures and low oxygen content: For fish species that require cool, oxygen-rich water, such as the rainbow trout (Oncorhynchus mykiss), it can be assumed that increasing water temperatures in aquacultures trigger stress in the fish and thus have negative effects on fish farming. The stress can lead to fish diseases and increased mortality. Plant growth is favoured in warmer water. The increasing biomass, when it dies, consumes the oxygen dissolved in the water during the decomposition processes. As a result, fish may experience oxygen deficiencies. For carp pond farming, rising water temperatures are actually likely to be beneficial, as the carp (Cyprinus carpio) is a warmth-loving fish and does not have excessively high demands on oxygen levels in aquaculture ponds. Higher temperatures lead to faster growth and thus to an increase in yield for fish farmers.

Stress due to extreme weather events: As a result of rising temperatures, heat periods and reduced rainfall, pond levels in aquaculture ponds decrease. Not only can high air temperatures cause the water to heat up even more quickly, but they can also lead to increased nutrient and pollutant concentrations, as substance inputs are less diluted. This can have a detrimental effect on the fish population. In extreme cases, premature emergency fishing may be necessary. By feeding cool oxygen-rich water into the fish farms during heat periods, water levels are kept high and water temperatures low. However, this requires a permanent availability of cool water. It becomes problematic for aquaculture farms when it is not possible to renew existing water rights or acquire new ones. It is unlikely that fish farms will be relocated to new sites on a larger scale, as the approval of new sites is also handled restrictively in many regions of Germany. If temperatures continue to rise, fish farming in aquacultures could become uneconomical in parts of Germany, as the supply of cool, oxygen-rich water becomes increasingly costly. But too much water, caused by continuous rain or heavy rain events, can also cause damage to aquacultures. Floods, flash floods, rising groundwater, overflowing sewers, landslides and silting can cause damage to ponds, buildings, machinery, feed and the fish stocks themselves.