Fish Thematic Assessment

Pressures on marine fish

Human activities translate into several pressures that act on marine fish in different ways across all OSPAR areas. (Human Activities Thematic Assessment).

Fish and shellfish harvesting activities are responsible for the highest pressure on marine fish, mostly through increased mortality in wild animals (either as target or by-catch). Pollution is the second highest pressure, with substances being introduced to the marine habitats that fish directly depend upon, from a variety of sources like shipping, cities and industrial activities, transport on land and waste treatment and disposal. The same activities are also responsible for putting pressure on marine fish through other mechanisms such as the direct input of litter and microbial pathogens.

Summary of human activities and relative pressures on fish by ecoregion:

1. Arctic Waters (Region I): Traditionally, fewer human activities are present in this area, given the relatively scarce human settlements. Nevertheless, pressures have been identified, mainly from sub-Arctic and temperate regions, the main ones being marine litter and the introduction of non-indigenous species, contaminating compounds and underwater noise. 
2. Greater North Sea (Region II): The main pressures acting on fish are selective extraction of species due to fishing, and physical loss or disturbance of substrate, mainly due to fishing activity (trawling and dredging) but also to the offshore infrastructure needed for renewable energy and navigational dredging.
3. Celtic Seas (Region III): In this area, the main pressures on fish arise from fishing (selective extraction of species, including by-catch), followed by inputs of contaminants (other substances) and marine litter.
4. Bay of Biscay and Iberian Coast (Region IV): Selective extraction of species due to commercial fishing (and to a lesser extent due to tourism and recreational fisheries) is the main pressure in this area. 
5. Wider Atlantic (Region V): Given the low proximity of this area to coastal and shelf areas (the only inhabited area here is the Azores) where human activities are concentrated, the Wider Atlantic is mostly impacted by the selective extraction of species due to commercial fisheries and, to a lesser extent, by the input of marine litter and other substances from shipping and military operations.

Confidence Assessment:

The anthropogenic pressures that affect fish are:

Extraction of, or mortality/injury to, wild species (by commercial and recreational fishing and other activities) [Biological]:
The impact of fishing mortality among target fish and shellfish species can cause reduced prey availability in other fish species, multi-level trophic cascades and phenotypic changes in the life history traits of fished populations, and also affect food web dynamics – which may or may not be sustainable – predator-prey relationships and competitive interactions within fish communities (Jennings and Kaiser, 1998; Thrush and Dayton, 2010; Woods et al., 2016). The mortality associated with fishing (a size-selective activity) restricts the age structure of fish communities, reducing the proportion of older and larger individuals (Bosch et al., 2022). The body size of fish thus decreases due to overfishing, and this affects the size composition of the fish community (OSPAR, 2017h). Ecosystem-based Fisheries Management (EBFM) can be enhanced to mitigate such impacts.

Selective extraction of species, including non-target catches [Biological]:
Fish and shellfish harvesting, either at professional or recreational level, can result in varying proportions of non-target fish by-catch. If the exploitation rates are unsustainable, this will have a negative impact on target and/or by-catch species. By-catch can refer to species of conservation concern like sharks, rays and skates, or to commercial species. The discarding of unwanted fish can lead to changes in species diversity, abundance and community structure (Jennings and Kaiser, 1998).

Physical disturbance to seabed (temporary or reversible) [Physical]:
Altering habitat structure can result in loss of habitat, and changes in fish and benthic species diversity, community composition, biomass, growth and productivity, thus potentially reducing available prey and affecting fish survival (Jennings and Kaiser, 1998; Watling and Norse, 1998; Thrush and Dayton 2002; Gill, 2005; Hiddink et al., 2006; Thrush and Dayton 2010; Woods et al., 2016; Gasparatos et al., 2017). Trawl fisheries, dredging, coastal and offshore infrastructure modify the seabed morphology and may lead to the re-suspension of sediments. Re-suspension and subsequent disposition of sediments in other areas can increase turbidity, inhibiting the settlement and growth of benthic species and thus altering prey availability (Jennings and Kaiser, 1998; Gasparatoset al., 2017).

Physical loss (to land or marine habitat) [Physical]:
Loss of habitat, particularly from fishing (Jennings and Kaiser, 1998; Woods et al., 2016), coastal and offshore developments (Gill, 2005; Gasparatos et al., 2017) and aquaculture (Naylor et al., 2000) can of course have a negative impact on fish and their prey. Such impact can be particularly important in the short and long term in areas that are routinely used by fish for spawning, feeding and as a nursery ground (see Disturbances of species below).

Changes to hydrological conditions [Physical]:
Hydropower infrastructure can alter water flow regimes, thereby affecting the migration routes of diadromous fish species to spawning grounds and can trigger complex community changes that differ upstream and downstream owing to varying effects on flow regimes (Gasparatos et al., 2017). Change of freshwater input linked to climate change or the presence of man-made structures in rivers can alter local coastal hydrogeological conditions, increasing salinity and stratification (Barange and Perry, 2009), which are important local conditions to which many fish species have become adapted.

Disturbance of species (e.g. where they breed, rest and feed) due to human presence [Biological]:
Disturbance leads to (temporary) habitat loss and higher energy expenditure (avoidance), with consequences for survival and reproduction in fish species (van Overzee and Rijnsdorp, 2014; Hawkins and Popper, 2017). Disruption of migration pathways and of access to breeding grounds for diadromous species can occur due to modification of watercourses (Gasparatos et al., 2017). The human activities that disturb fish species, due to their presence, are:

• coastal development involving the physical restructuring of rivers, coastline or seabed (water management), tourism and leisure infrastructure and nuclear energy;
• offshore construction for the production of energy (with offshore wind farms occupying a large surface area);
• ship traffic in any form (including fisheries);
• tourism and leisure activities (e.g., recreational boating at sea, sailing, recreational fishing, snorkelling, diving).

Input or spread of non-indigenous species [Biological]:
Invasive species may be dispersed and introduced into new marine environments through international shipping, fishing, aquaculture, the aquarium trade and canal construction / operation (Woods et al., 2016). Invasive species can affect food web dynamics, displace native species, introduce diseases and lead to changes in habitat type, thereby significantly altering marine ecosystems (Crain et al., 2009; Gestoso et al., 2018). The pressure exerted is assessed in more detail in the Non-indigenous Species Thematic Assessment. 

Input of microbial pathogens [Biological]:
The anthropogenic activities that can facilitate the pathogenic invasions that can deplete wild fish stocks include aquaculture (including shrimp and salmon farming) (Naylor et al., 2000), invasive species (Crain et al., 2009) and marine litter (Amaral-Zettler et al., 2020).

Input of nutrients - diffuse sources, point sources, atmospheric deposition [Substances, litter and energy]:
At local scale, inputs of nutrients from farming run-off, dredging, aquaculture and sewage releases can have negative impacts on fish species by causing eutrophication (Naylor et al., 2000). The installation and decommissioning of offshore energy developments can also cause contaminant remobilisation (Gill, 2005; Gasparatos et al., 2017). Eutrophication-induced hypoxia (oxygen depletion) can have detrimental effects, particularly on benthic fauna (lethal or sub-lethal) and bottom-dwelling fish (Crain et al., 2009; Woods et al., 2016). The negative impacts can include reduced benthic diversity and altered species composition, as well as changes to growth and reproduction, physiological stress and changes to migration patterns in mobile species.

Input of litter (solid waste matter, including micro-sized litter) [Substances, litter and energy]:
The introduction of litter, whether land-based (rivers, industrial sources, tourism) or marine-based (shipping, fishing, aquaculture, tourism) can cause ingestion and entanglement, leading to reduced food consumption or predator avoidance, injury, suffocation or death (Woods et al., 2016). Input of litter into the environment can also facilitate the introduction of non-native species, diseases or toxic substances that can negatively impact fish species.

The input of litter and the consequent pressure on fish is linked with environmental impacts in the Marine Litter Thematic Assessment. 

Input of other substances (e.g., synthetic substances, non-synthetic substances, radionuclides) - diffuse sources, point sources, atmospheric deposition, acute events [Substances, litter and energy]:
Synthetic substances, non-synthetic substances and radionuclides can cause physical harm to fish. Fish populations in freshwater environments suffer the most acute threat of chemical contamination, for example local population extinction through metal contamination resulting from industrial or mining activities (Hamilton et al., 2016). In the marine environment, chemical contamination is a threat in coastal fisheries where pollutant levels are highest, particularly from oil run-off, production and transportation; petroleum; heavy metal contamination; and synthetic substances from agriculture and urbanisation (e.g., pharmaceuticals in sewage) (Hamilton et al., 2016). Food-chain transfer and bioaccumulation of pollutants in fish tissues (e.g., metals, persistent organic pollutants and organochlorine contaminants) can adversely affect fish in all aquatic environments, impacting reproduction, development and/or survival of offspring and causing population decline (Hamilton et al., 2016). 

Inputs of other substances and their pressures on fish species are closely linked with environmental impacts in the Radioactive Substances Thematic Assessment and the Hazardous Substances Thematic Assessment.

Input of anthropogenic sound (impulsive, continuous) [Substances, litter and energy]:
The activities that emit anthropogenic noise into the environment are shipping, military operations and construction. Anthropogenic noise is a form of energy that can result in behavioural impacts, physical and/or physiological impacts, masking and death (Gill, 2005). The behavioural impacts can be displacement and changes in swimming patterns and migration; these can affect fitness, survival and reproductive success. Masking due to anthropogenic noise makes it difficult to detect biologically important sounds, which can result in reduced survival for fish species. The physical and/or physiological impacts are temporal threshold shift (TTS) or permanent threshold shift (PTS) (Hawkins and Popper, 2017). 

Inputs of noise and their pressures on fish are linked with the environmental impacts in the Underwater Noise Thematic Assessment.

Input of other forms of energy (including electromagnetic fields, light and heat) [Substances, litter and energy]:
Offshore renewable energy operations emit electromagnetic fields (EM) into the environment that can have behavioural impacts (attraction/avoidance) on EM field-sensitive species (particularly elasmobranchs) (Gill, 2005; Gasparatos et al., 2017). These behavioural impacts can include displacement and changes in swimming patterns and migration, which in turn affect fitness, survival and reproductive success. The increase in average ocean temperatures driven by greenhouse gas-induced climate change can alter growth, reproduction, behaviour, migration patterns, spatial abundance and distribution, with the potential to cause community-level changes (Woods et al., 2016).

Changes in temperature caused by climate change and the pressure on fish are linked with the Climate Change Thematic Assessment.

Input of water - point sources (e.g., brine) [Substances, litter and energy]:
Input of brine can occur as a by-product of industry (e.g., oil and gas extraction, chemical plant waste, desalination, irrigation run-off) and changes to salinity regimes can result from altered tidal exchange, drainage alterations, freshwater diversions, and global warming. Estuaries are particularly vulnerable to changes in salinity owing to slow water movement and close proximity to the drivers affecting salinity. Salinity changes can result in mortality or sub-lethal stress in fish and other benthic species and potentially lead to changes in community and ecosystem structure (Crain et al., 2009; Gasparatos et al., 2017), and can also have an important influence on the development and growth of many fish species (e.g., egg fertilization and incubation, yolk sac resorption, early embryogenesis, swim bladder inflation and larval growth) (Boeuf and Payan, 2001).