The Significance of Fisheries

Seafood is an important part of the world's food supply. Only about 65% of production from marine capture fisheries is fed directly to people. The rest is fed to cattle, poultry, and other fish. Per capita production of food fish is about 10 kg and has remained at roughly this level since the 1970s, although if growth in Chinese fishing is ignored there has actually been a decline. Global marine fishery production reached about 80 million tons in the mid 1980s, a quadrupling since 1950, and then has generally leveled off. If growth in Chinese production is removed from the total, production dropped about 10% during the 1990s. Marine capture fisheries represent roughly 70% of total world fisheries production, which includes aquaculture. The total value of the global fish trade in 1999 was about US $53 billion.

According to the FAO, in 1998 about 36 million people were employed in fishing and aquaculture, about 15 million of whom were full time workers. Roughly 60 percent of fishers were employed in marine fisheries, mostly in small scale fisheries. It is estimated that more than 100 million people "are directly dependent upon fisheries for food, income and livelihood." (Garcia and de Leiva Moreno, 2003) A 2001 study calculated that there are 51 million fishers in the world, the vast majority of whom live in developing countries. Virtually all of these people are employed in small-scale fisheries. More than 25,000 people die fishing every year.

Source: Garcia, S. M., & de Leiva Moreno, I. (2003). Global Overview of Marine Fisheries. In M. Sinclair, & G. Valdimarsson (Eds.), Responsible Fisheries in the Marine Ecosystem (pp. 1-24). Oxford: CABI.

Fish is the primary source of protein for one-sixth of the world's population. It is proporionally more important in the developing world. For example, thirty percent of the animal protein consumed in Africa comes from fish. Global sustainable marine fish catch is estimated at 69 to 96 million metric tons. Catch leveled off at roughly 100 million metric tons in the 1990s, but global production will have to reach 140 million tons by 2025 to meet the demands of a growing population. Aquaculture could help meet this need, but it is unclear if it can be productive enough.

Source: Helfman, G. S. (2007). Fish Conservation. Washington: Island Press.

Ecosystems and Sustainable Fishing

Overfishing damages entire ecosystems, not just the target stock. In response, some scientists have advocated focusing management on the ecosystem in which fishing is done instead of directly on the species being fished. "EBFM implies several objectives for protecting ecosystem attributes from ecosystem-level effects of fishing: (1) to maintain predator-prey relationships, energy flow and balance, and diversity (Livingston et al. 2005); and (2) to balance diverse societal objectives, by taking into account the knowledge and uncertainties about biotic, abiotic, and human components of ecosystems and their interactinos within ecologically meaningful boundaries (FAO 2003; Garcia and Cochrane 2005)." (Defeo et al. 10) However, ecosystem based fisheries management has significant difficulties, including a lack of clear ways to distinguish the effects of current fishing from past fishing, natural variability, and other environmental changes. Another problem is the lack of ecosystem data for "artisanal fisheries in poor countries".

Source: Defeo, O., McClanahan, T. R., & Castilla, J. C. (2007). A Brief History of Fisheries Management with Emphasis on Societal Participatory Roles. In T. McClanahan, & J. C. Castilla, Fisheries Management: Progress Towards Sustainability (pp. 3-21). Oxford: Blackwell.

Single-species models of fish populations are not good enough for many purposes, particularly considering that there is evidence that overfishing has caused entire ecosystems to collapse. A relatively recent innovation, multi-species models are substantially more complex than single-species models. A major issue is how to use data to estimate parameters in the models.

Single-species models vary in complexity, but all "are of the form of an internal black box, which simulates an ecosystem based on some parameters" (Stefansson 173). Statistical methods must be used to estimate the parameters, and these methods are not always very good, to the point of causing the models to make huge errors. However, the models are not useless and in many cases make unsurprising predictions, such as that high fishing mortality may cause fish stocks to collapse.

The hardest part of creating a multi-speces model is figuring out what effects are important to include. Frequently, the effects that needs to be modeled are predator-prey relationships. The models must also often include migration, growth, spatial factors, and fishing fleet behavior.

Real-world "experiments" such as the closure of fisheries after collapse show that single-species population dynamics is basically correct. But more complex multi-species models are more difficult to verify.

In general, fish stocks do not collapse because of a policy of fishing beyond MSY. Instead, a policy of at or below MSY fishing is not followed. In most cases, it would be fine to fish at MSY as long as it could be guaranteed that it would not be exceeded. But achievement of this goal is complicated by of uncertainty about the stock's present status, how much fishing the stock can take, and lack of understanding of the interactions between the stock and other species.

Sometimes, the models suggest that overfishing a predator could boost a depleted prey stock. Whether this is a good idea is questionable.

"The precautionary approach, stated in its simplest form, implies that care needs to be taken to ensure that fishing is undertaken in a sustainable manner and that when uncertainty is present, this should be taken into account by reducing fishing mortality. In implementing the precautionary approach, reference points have been defined. Loosely, they are defined in order to set rules that satisfy the criterion that as long as fishing is within bounds defined by the reference points, fishing mortality will not exceed specified harvest rates." (Stefansson 177)

Models that suggest fishing should be increased should be interpreted with caution and their predictions should be acted on only if the multi-species model is clearly better than other models. But in general, multi-species models support the idea that fishing mortality should be minimized. There may be cases where an increase in fishing could make a fishery more sustainable, but this must be shown on a case-by-case basis.

Multi-species models can help illuminate the failures of various fishery management techniques. Some examples:

  • A quota system does not distinguish the ages of fish caught, allowing heavy fishing of young fish which leads to collapse.
  • Under an effort-control system in a multi-species fishery, effort is unevenly distributed over the species, causing unsustainable fishing of one or more of them.
  • A closed area fails because an excessive amount of fish can be caught outside the area.

Minimizing fishing pressure is a good idea, but multi-species models suggest that achieving this goal is harder than expected. Sustaining fisheries requires using several different management techniques together. Although the models sometimes require data which is not available, they can also be used to help determine what kinds of data are necessary to make good management decisions. Management measures must be adopted that will work even despite uncertainty.

Source: Stefansson, G. (2003). Multi-species and Ecosystem Models in a Management Context. In M. Sinclair, & G. Valdimarsson (Eds.), Responsible Fisheries in the Marine Ecosystem (pp. 171-188). Oxford: CABI.

Coastal eutrophication assessment in the United States

Eutrophication is a growing problem along US coasts. Excess nutriets (i.e. nitrogen/phosphorous) added by humans to the water disrupts the ecosystem leading to algal blooms and a lack of biodiversity. Nitrogen levels have increased 4 to 5 fold, and 10 fold in some areas. The result of eutrophication is hypoxic/anoxic water (i.e. not enough oxygen in the water) and can disrupt the ecosystem. Solutions have to come mostly from the local level.

Indicators for eutrophication include:

    1. Algal blooms, as measured by the presence of excess chlorophyll a

    2. Micralgae and epiphytes, which tend to suffocate bivalves.

    3. Toxic Blooms

    4. Nuisance Blooms: excessive numbers of small organisms that clog filter feeder siphons.

    5.  Low or 0 dissolved oxygen concentration

http://www.springerlink.com/content/j17m6431q7860777/?p=3079f9b2e21f4c52b2b2448d0f6d7057π=4

Integrated Coastal Zone and Fisheries Ecosystem Management: Generic Goals and Performance Indicies:http://www.jstor.org/view/10510761/di014619/01p0061x/0?frame=noframe&userID=12f70677@mit.edu/01c0a8346600501cc5874&dpi=3&config=jstor

    Biodiversity isn't enough to report how effective policies are in maintaining ecosystem. Overfishing in coastal zones have caused a shift from long-lived to short-lived species, from mature to immature individuals, and from higher to lower trophic levels. Thus we need some indices to gage how well fishery management techniques achieve the goal of reversing such trends.

    To be sustainable we  must look at the six principles of ecologically sustainable development:

          1. Improvement in material and nonmaterial well-being

          2. Equity between generations

          3. Equity within generations

          4. Maintenance of ecological systems and biodiversity

          5. Global issues, such as international trade and cooperation

          6. Dealing cautiously with risk, uncertainty, and irreversibility of impacts.

   There are three main areas of management:

         1. Fishery Management, aimed at "optimizing catch and effort for a particular species of interest" and minimizing bycatch.

         2. Coastal zone Management, aimed at "habitat protection for catch and bycatch" and "maintenance of water quality."

         3. Catchment Management, aimed at minimizing "deleterious runoff inter water courses.

There are four indices that can help gage whether we are achieving the goals of maintaining trophic balance/conserving endangered species/allowing for succession to occur efficiently/establishing an ecosystem that is composed of the proper balance of species

         1. Trophic structure index: Each trophic level is weighted by a factor of ten (each trophic level is weighted ten times the level below it). This follows the "10% rule of thumb for ecological efficiency of transfer between trophic levels." That is each trophic level should have approximately one tenth the biomass of the one below it. (See source for mathematical index). Because trophic structures and biomass vary from region to region, an absolute score isn't very helpful in comparing regions. However it could be used to track trends.

         2. Conservation Status index: Measures how well endangered species specifically are being conserved in a given ecosystem.

         3. Species Composition Index: Measurement of whether the ecoystem protects unique species or whether it is representative of common species.

         4. Bioconstruction index: How well the ecosystem represents the stages of succession and the replacement time.

Fish Biology and Fishery Management

The studies of fish physiology and life history must be brought together and then used to help manage fisheries. Examples from the Pacific salmon fishery are the use of physiological knowledge to predict migratory mortality and to thus adjust catch targets accordingly, and helping minimize mortality as the salmon attempt to get around dams on their journey upstream.

Additionally, physiology and life history knowledge is necessary to understand the impacts of environmental change on fisheries. For example, physiological studies of the effects of temperature change on fish are important to understand the impacts of climate change. Physiology is also necessary to understand how pollutants affect fish.

Fish physiologists and fishery ecologists do not often work together, although they are studying the same species. It has been suggested that the lack of attention to fish physiology has been in part because of focus on the evolutionary aspects of life history.

Source: Young, J. L., Bornik, Z. B., Marcotte, M. L., Charlie, K. N., Wagner, G. N., Hinch, S. G., et al. (2006). Integrating physiology and life history to improve fisheries management and conservation. Fish and Fisheries , 7 (4), 262-283.

The temperatures of maximum growth rate and feed conversion efficiency of Atlantic cod vary by size of the young fish, both decreasing as the fish get larger. This may help explain cod migration, as well as having implications for fish farming. Fish can select temperatures for maximization of growth rate or growth efficiency.

Source: Imsland, A. K., Foss, A., Folkvord, A., Stefansson, S. O., & Jonassen, T. M. (2006). The interrelation between temperature regimes and fish size in juvenile Atlantic cod (Gadus Morhua): effects on growth and feed conversion efficiency. Fish Physiology and Biochemistry , 31 (4), 347-361.

Fish biology is also necessary to understand what happens when fish that are caught are returned to the wild.
Many sharks, tunas, and marlins are captured every year but are required to be released because of size and bag limits. The fate of these fish after they are released is not well known. The effect of the stress of capture on blood pH, for example, is easily measurable. However, it is difficult to study post-capture mortality because such fish cannot readily be held in captivity or confined for study purposes.

Therefore, various kinds of tagging studies are necessary. Studies indicate low short-term mortality for sharks and tuna but have produced variable results with billfish. Other studies have attempted to link level of injury and type of hook used for capture with subsequent mortality. It may be possible to use blood chemistry and tagging studies to determine the cause of death of fish that do not survive.

In general, information is lacking in this area. There is some evidence that physical injury (rather than physiological stress) is the more important factor in determining mortality. Further study is necessary to clarify the relationships between injury, stress, and mortality so that measures can be devised to increase survivorship of released fish. Additionally, there is little information on the population-level effects of catch and release. Studies to address this must involve more fish and longer time periods, and will become more feasible with advances in tag technology.

Source: Skomal, G. B. (2007). Evaluating the physiological and physical consequences of capture on post-release survivorship in large pelagic fishes. Fisheries Management and Ecology , 14 (2).

Fishing activities can also drive the evolution of fish. For example, there is evidence that fishing has caused some fish to evolve so that they mature earlier. Fishing selects for early maturity because late maturing fish are caught before they can reproduce, removing their genes from the gene pool. Such changes cannot be readily reversed.

Source: Helfman, G. S. (2007). Fish Conservation. Washington: Island Press.

Egg quality (that is the rate by which eggs survive and hatch) is dependent on a variety of internal (genetic) and external (developmental) factors. These include the following:

    1. Hormones present: Whether farmed or in the wild, the fish needs a supply of regulatory materials (hormones) from the mother because the egg cannot create many of these on its own in situ. In eggs there exists a significant amount of cortisol, whose concentration decreases as the egg approaches hatching. Exposure of juvenile and adult fish to such amounts of cortisol has harmful effects. Also, the mother passes several sex steroids to the egg. Often there is contradictory or insufficient knowledge about what effects certain hormones have on egg quality. People have been trying different ways to induce ovulation in fish using hormones. Some methods produced eggs of low quality. However, "administration of GnRHa via controlled-release delivery systems, which stimulated long-term elavultions of plasma GtH II, have been successfully used to induce ovulation of captive white bass and striped bass without apparent detrimental affects on survival to hatching." 

    2. Egg size: Usually thought that larger eggs are of higher quality, but this is not always the case. Larger hatchlings from larger leggs have a survival advantage in the first few days after beigng hatched because they have higher growth rates, larger yolk reserves, are able to avoid predators better, and can eat more variety of food. However, large eggs are at risk as being noticible as prey and because they take longer to hatch could possible make them more vulnerable to adverse conditions. In some fish, such as rainbow trout, egg size does not appear to be an important indicator of egg quality because the size advantage of larger eggs was "soon masked by other environmental determinants of growth." In Atlantic cod, however, egg size and fecundity are closely related.

 3. Age of Fish: Many types of fish produce higher quality eggs during their second seasong of spawning rather than their first.

 4. Environment: Egg quality is generall higher in wild fish compared with captive fishery stocks, probably because of the diet of the broodfish and other factors such as temperature, salinity, pH, and photo period.

          Diet: Egg quality is influenced by lipid composition. Eggs with a higher content of n-3 fatty acids tend to be of better quality.Also, carbohydrate and protein composition affect egg quality, but not to the extent that lipids do. Fish fed natural diets tend to produce higher quality eggs that fish fed commerical diets in aquaculture.

           Photoperiod: Photoperiod manipulation is used to delay or advance spawning.

           Water temperature: Temperature influences "metabolism, activity, and structure of the developing embryo. Incubation temperature that are too high or too low can lead to egg death in early development stages.

            Captivity: Crowding and stress associated with confinement reduces egg quality. Often, there are low fertilization rates, odd spawning intervals due to the stresses on broodfish in captivity. "Both chronic confinement experienced during the final stages of reproductive development, and periods of acute stress, have been shown to disrupt the endocrinology underpiinning normal growth and development of the ovary in trout, and may result in significantly lower progeny survival rates. Also, fish in captivity often don't release their eggs and their eggs over-ripen in the body cavity.
Egg quality in fish: what makes a good egg? 

Management Methods

The effectiveness of various fishery management systems was analyzed by the OECD in 1997. The management methods can be divided into three types: input controls, output controls, and technical measures. "Output controls include total allowable catch (TAC) (total quotas), IFQs, and vessel catch limits." (Sutinien and Soboil 292). Input controls are various kinds of restrictions on the ways in which fishing may be conducted, such as effort limits and restrictions on gear. Finally, technical measures include restrictions on when and where fishing can be done and what sizes and sex of fish may be taken.

The evidence suggests that individual fishing quotas were the most effective management system. IFQs, which give individual fishers the right to take particular amounts of fish, had numerous benefits. They

successfully kept the catches in fisheries where they were used below the total allowable catch in most cases, and also were largely effective in eliminating the race to fish created by a TAC quota only system.

A TAC-only system creates a race to fish by limiting the number of fish available to the participants in a fishery. Therefore, to maximize individual revenue, fishers must fish as much as possible until the quota has been reached. This causes numerous problems: They include wasteful and dangerous fishing practices, excessive investment, and lower quality of the final product.

In contrast, there is no incentive to race under an individual fishing quota because fishers are no longer competing with each other for shares of the catch.

IFQs have a significant problem: Deciding how to distribute the quotas when the system is set up. Although the various other possible management techniques don't have this problem, "OECD (1997) concluded that none of the other (non-IFQ) management measures performs well when used without IFQs, i.e. they do not effectively control exploitation and mitigate the race-to-fish." (Sutinen and Soboil 295) Most of the other techniques also increased enforcement costs.

Source: Sutinen, J. G., & Soboil, M. (2003). The Performance of Fisheries Management Systems and the Ecosystem Challenge. In M. Sinclair, & G. Valdimarsson (Eds.), Responsible Fisheries in the Marine Ecosystem (pp. 291-309). Oxford: CABI.

A new proposal for management of the fish stocks off the Northeastern United States has been advanced by the Northeast Seafood Coalition, an industry group. Instead of the current management system, the days-at-sea input control method, many of the species in the Northeast fishery would be managed under a points system. Each fishing vessel would be awarded points to be "spent" on catching fish. Each kind of fish would cost a specific number of points. During the fishing season, data would be collected on what fish have been caught and a computer model would adjust the point values of various species, making them more or less expensive depending on how much of the total allowable catch of each species had been taken.
The scheme is described in considerable detail in documents attached to this page.

Technology Issues

The status of the fishing fleet is not as well documented as it could be. Although the fleet size appears to have been fairly stable since 1980, technological improvements mean that actual fishing capacity has probably more than doubled since then. Innovations have also made fishing safer and improved fishery regulation enforcement.

Source: Garcia, S. M., & de Leiva Moreno, I. (2003). Global Overview of Marine Fisheries. In M. Sinclair, & G. Valdimarsson (Eds.), Responsible Fisheries in the Marine Ecosystem (pp. 1-24). Oxford: CABI.

The number of "large vessels" in the global fishing fleet went from about 600 thousand to 1.2 million between 1970 and 1990. This total ignores vessels used in subsistence fishing.

Source: Helfman, G. S. (2007). Fish Conservation. Washington: Island Press.

It has been suggested that an improved understanding fish taste could facilitate the use of effective bait in commercial fisheries, allowing the increased use of fishing techniques such as longlining and traps to replace environmentally destructive trawling. 

Source: Kasumyan, A. O., & Døving, K. B. (2003). Taste preferences in fishes. Fish and Fisheries , 4 (4), 289-347.

Improvement of Aquaculture

The main problem of aquaculture is the adverse effects of a fish farm on its surrounding environment. Such issues as spread of disease, risk of escapes, and the seepage of nitrogenous wastes into surrounding ecosystems. The Convention on Biological Diversity published a paper for the World Fisheries Trust addressing some changes that would improve aquaculture. These include:

  • change nutrition - develop alternative protein sources (plant-based); use herbivorous fish in farms
  • reduce waste - use plant-derived protein (contains less phosphorus); correct amino-acid balances in feed; reduce feed conversion ratio; improved filtration/fallowing of wastes
    • extrusion = exposing raw plant-based feeds to high pressure and heat, followed by rapid lowering of pressure
      • improves digestibility of feed, yields high protein levels, inactivates "antinutrients", float better (easier to eat)
    • produce feed supplements using recombinant DNA (more economical)
  • culture species together (polyculture) - waste of one species used as feed for another species (long history in freshwater)
  • contain cultured animals - computer simulation models to compute stocking density
    • closed systems - recycled-water plants allow for higher stocking densities, less disease, fewer breakdowns, lower operating costs
    • indoor systems - higher operating costs, increased freshness of fish, higher price (fewer pollutants)
    • contained saltwater systems - reduce discharge and disease transmission
  • reduce dependency on wild seed
  • reduce use of chemicals additives and treatments - prevent pests/diseases from being problems (lower stocking densities, higher water flow)
  • reduce disease transmission - reduce exposure to pathogens, reduce stress levels (high ammonia & low oxygen levels), biological control of diseases and parasites, implement strict health and quarantine protocols and policies

temctre-01-02-en.pdf

Fish Genetics

Although there is much research on unique populations of individual species, not enough about the genetics of fish is known. Scientists are still researching the effects of genetic drift versus natural changes in allele proportions.

Gene Flow in Perch.pdf

Pollution 

Pollution is one of the main environmental factors that adversely affects fish.  In particular, air pollution from factories and cars, as well as runoff, is disastrous for ocean health.  Ocean pollution harms fish populations by harming the fish directly and by harming their plant food sources.  This destroys fish populations, reducing the health of the populations, and also reducing the amount of fish that we can remove from the oceans.  If populations are healthier, we will be able to fish more, so reducing pollution should be a priority.  Also, polluted fish waters lead to fish with pollutants, such as lead and mercury.  These pollutants harm the health of the other fish and the humans that eat them.  In order to improve the health of fish populations and the health of the humans consuming fish, regulating pollution ought to be a top priority.  Increased reliance on energy sources such as solar and wind power would reduce the demand for coal plants.  Reduced coal plants means cleaner air, which means cleaner oceans and healthier fish populations.  Source: Effects of Pollution on Fish. Lawrence, A.J. and K.L. Hemingway.  Blackwell Pubishing, 2003.

Aquaculture

If we tend to use aquaculture as a way of rebuilding populations and not only providing a current food source for people, we must consider the fact that farm-raised fish are not as viable in the wild as are normal fish.  This is largely due to the fact that the farm-raised fish are accustomed to a pampered lifestyle with regular feeding times and a lack of predators.  So, in order to make aquaculture more viable as a means of replacing depleted populations, we should consider making the aquaculture environments less hospitable, and more representative of the natural environment.  This would make the released fish more adapted to the challenges of the wild and thus more likely to survive.  A few possible ways of making aquaculture more representative of the natural environment might be adding predators and making feeding more erratic and competitive. Source:  "Tanks teach cod to fend for themselves"  Nature News, 2005. 

Dependence on Fish

Some countries in the world depend on fish.  Many poor nations have no other resources or no other geographically feasible ways of feeding their populations.  But the world's richest nations are not among those countries.  The U.S. and Western Europe, which consume a great deal of fish, do not depend on fish to survive.  Fishing clearly needs to be reduced.  It is not equally feasible for all nations to do this, so those that can reduce fishing are the ones that should.  It is disgustingly arrogant to assume that all countries will follow fishing reductions that the US proposes, especially since the US has an atrocious record of accepting international agreements aimed at helping the environment.  So, the US cannot tell the rest of the world what it should do, but it should take the responsible step of reducing fishing, since that is something that we are capable of doing without starving the nation.  Perhaps it is unreasonable to hope that the US would proactively take action to help the entire world, but, with enough public pressure placed on Congress, this could happen.  The reduction in fishing by major nations that can afford to obtain food from other sources would alleviate the stress on the oceans enough so that smaller nations dependent on seafood would be able to continue their traditional practices.

Species Variability

While some fish species are suffering immensely, others are thriving. For example, as the fishermen in Gloucester said, dogfish are doing extremely well.  When one species controls a disproportionate amount of the resources in an ecosystem, the other species naturally suffer, since the resources in an ecosystem are limited.  In order to help the less successful species persist, a potential solution might be to encourage the fishing of competing but more abundant species. These other species may not be as popular in the market, but with government subsidies, it could be financially feasible to fish these species.  Governments already subsidize fishing, so stipulating that certain species be caught as part of the subsidies might make the subsidies more effectively increase the abundance of overfished populations.  Even if the market does not demand any of the more abundant species, reducing their numbers could benefit the depleted populations by increasing the amount of resources directly available to them. 

Temperature's Effect on Fish
On fish immunology:

Fish generally respond faster and with a greater magnitude to diseases when the temperature is higher in the "physiologically tolerated range."  At lower temperatures, fish may suffer from a complete lack of response to the disease.  However, if the temperature is high enough for the primary response to occur, then the secondary response may be at a lower temperature with the same results.  Also, after the first few days of being exposed to the disease, the antibody count become independent of temperature.  Fish can also acclimate to a lower temperature with the only difference being a slight change in their antibody count.  Overall, a higher temperature can greatly enhance a fish's immune system's ability to "remember" a disease, or make effective memory cells and antibodies in case of another infection in the future.

Microbial diseases of fish / edited by R.J. Roberts.    London ;   New York :   Published for the Society for General Microbiology by Academic Press,   1982.

On younger fish:

Temperature can affect a population's natural regulators/density dependent effects

   1.  El Niño generally leads to warmer water, which then leads to an increase of the fish that survive, but it also leads to an increase in canabalistic parents.
   2. Cold water slows development and the larva are carried closer to shore.  Cold water also kills predators on the nursery floor and favors the growth of food specifically needed by the younger fish.

Definition of a Fish Population vs. Fish Stock

    A population of fish is one which is a single-species, continuing reproductive unit.
    A fish stock is the "fished part of one or more population."  Basically, fish stock is really just the fish found in the area.

 Fish populations are not defined by genetics because the fish will often mix with other populations. Also, there have been genetic separations in the same population. In contrast, two completely separate populations may have very similar DNA. Similar populations in the same location often appear to be the same population, but can be separated out by their spawning cycles.
 Natural Regulation of Single-Species Populations

Fish populations are perfectly capable of regulating the size of their own population.  Usually, such density dependent processes happen only on a local scale and are "much influenced by spatial heterogeneity of the population and environmental factors." Density dependent regulation generally includes predation/canabalism, infection, competition, and changed reproductive processes.  Younger (baby) fish are the most affected by competition. Temperature also affects regulation (see Temperature's Effect on Younger Fish). Regulation for adult fish is affected by changes in growth rate, and changes of age at first maturity. When the adult population decreases (not due to overfishing), the growth rate increases and the fish reach maturity at a younger age.  Since fecundity is proportional to weight, the changes compensate for the loss in the number of eggs that could have been.

 Dahlem Workshop on Exploitation of Marine Communities (1984 :    Exploitation of marine communities : report of the Dahlem Workshop on Exploitation of Marine Communities, Berlin, April 1-6, 1984 / editor, R.M. May ; rapporteurs, J.R. Beddington ... [et al.].    Berlin ;   New York :   Springer-Verlag,   1984.

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