Fish mortality

Fish mortality is a term widely used in fisheries science that denotes the loss of fish from a stock through death. The term is also commonly used in British English as a synonym for fish kill. Fish mortality can be divided into two types:

• Natural mortality: the removal of fish from the stock due to causes not associated with fishing. Such causes can include disease, competition, cannibalism, old age, predation, pollution or any other natural factor that causes the death of fish. In fisheries population dynamics natural mortality is denoted by (M).^[1]

• Fishing mortality: the removal of fish from the stock due to fishing activities using any fishing gear.^[1] It is denoted by (F) in fisheries models. (M) and (F) are additive instantaneous rates that sum up to (Z), the instantaneous total mortality coefficient; that is, Z=M+F.^[2] These rates are usually calculated on an annual basis.

Estimates of fish mortality rates are often included in mathematical yield models to predict yield levels obtained under various exploitation scenarios. These are used as resource management indices or in bioeconomic studies of fisheries.

Contents 1 Estimating mortality 1.1 Natural 1.2 Fishing 2 Why estimating mortality is important 3 See also 4 References

Estimating mortality

Natural

Estimating natural mortality (M) is one of the most difficult and critical elements of a stock assessment (Hewitt et al. 2007). There are two basic approaches used to estimate natural mortality: tagging studies and growth parameters. Tagging studies are used in the Brownies Model, where multiyear tagging studies are used to estimate natural mortality based on recaptures:

fi = ri(1 – Si)

The Paulys Model (using growth parameters) is an indirect way of estimating natural mortality. It assumes that there is a relationship between size and natural mortality. Pauly’s original method was based on the correlation of M with von Bertalanffy growth parameters (K and L∞) and temperature (Gunderson 2002):

N0 = N 1*e(-Z*∆t)

In the Hoenigs method, M was inversely correlated with longevity across a wide variety of taxa (Hewitt and Hoenig 2005):

Ln(M) = 1.44-0.982*ln(tmax)

Fishing

Fishing mortality (F) can be estimated by dividing the catch by the mean stock size. The catch includes annual commercial and recreational landings, along with dead discards. Bycatch discards would be estimated by estimating the percent of fish that are captured in a certain gear and the mortality associated with being captured in this gear. These mortality studies are generally done by using cages for a certain period of time following capture to determine the percentage of fish that die during being held in holding cages. These deaths are assumed to be associated with physical injury or physiological stress from being captured in the gear used during capture.

Why estimating mortality is important

Mortality estimates are important to managers. Determining mortality rates are critical for determining abundance of fish populations. Using the model Z=M+F with M being Natural mortality and F being Fishing mortality (combined mortality from landings plus discard mortality) you can estimate the trend of a population. The mortality rates give you the total deaths of a population when you compare these to the total births or recruits to the population, you can determine if a population is increasing or decreasing. Knowing these rates can help managers to set harvest limits to (MSY) maximum sustainable yield or (OSY) optimum sustainable yield to give the maximum benefit to the stakeholders of the resource.

See also

• Mortality rate
• Population dynamics of fisheries
• Fish kill

References

Fisheries science and wild fisheries Fisheries science Population dynamics of fisheries Shifting baseline Fish stock Fish mortality Stock assessment Fish measurement Fish counter Data storage tag Catch per unit effort Biomass Fisheries acoustics Acoustic tag GIS and aquatic science EcoSCOPE Age class structure Trophic level Trophic cascades Match/mismatch hypothesis Fisheries and climate change Marine biology Aquatic ecosystems Bioeconomics EconMult Ecopath FishBase Census of Marine Life OSTM Fisheries databases Institutes Fisheries scientists Wild fisheries Ocean fisheries Diversity of fish Coastal fish Coral reef fish Demersal fish Forage fish Pelagic fish Cod fisheries Crab fisheries Eel fisheries Krill fisheries Kelp fisheries Lobster fisheries Shrimp fisheries Eel ladder Fish ladder Fish screen Migration Salmon run Sardine run Shoaling and schooling Marine habitats Marine snow Water column Upwelling Humboldt current Algal blooms Dead zones Fish kill   Fisheries management, sustainability and conservation Management Fisheries management Monitoring control and surveillance Vessel monitoring system Fishery Resources Monitoring System Catch reporting Fisheries observer Illegal, unreported and unregulated fishing Fisheries organizations Quotas Catch share Individual fishing quota Minimum landing size Discards Bycatch Cetacean bycatch Turtle excluder device Shrimp-Turtle case EU quotas EU MLS Exclusive economic zone Sustainability Sustainable fisheries Maximum sustainable yield Sustainable seafood Overfishing Environmental effects of fishing Fishing down the food web Destructive fishing practices Future of Marine Animal Populations The Sunken Billions End of the Line Conservation Marine Protected Area Marine reserve Marine conservation Marine conservation activism Salmon conservation Grey nurse shark conservation Shark sanctuary Organisations Marine Stewardship Council Friend of the Sea SeaChoice Seafood Watch Oceana Sea Around Us Project WorldFish Center Defying Ocean's End HERMIONE PROFISH International Seafood Sustainability Foundation Sea Shepherd Conservation Society Greenpeace Related issues CalCOFI Marine pollution Mercury in fish Shark finning List of fishing topics by subject Index of fishing articles Fisheries glossary

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