This is a continuation of a previous article in which aspects of fisheries biology and ecology pertinent to fisheries management were addressed briefly. We reiterate that although certain information on fish biology and life history is important and relevant, other areas of biological knowledge which are of interest to natural scientists are rarely used in fisheries management. Fisheries scientists should ideally not indulge in research which, though interesting, is very expensive and has limited or no relevance to fisheries management.

In South Africa it was suggested at a prominent scientific conference that although 800 international quality scientific papers were produced under the auspices of the Benguela Ecology Programme (the BEP), only a few were ever used or contributed to the scientific process leading to the determination of the annual total allowable catch (TAC) and other fisheries management decisions. The intention of this preface is not to minimise the importance of general biological research, but rather to emphasise the importance of clear objectives and timetables for research funded for fisheries management purposes, and to distinguish this research from similar academic research.

Natural mortality versus fishing mortality

Regardless of the availability of numerous sources of energy in the ecosystem, all living organisms eventually die. All causes of death of marine organisms which are not man-induced are regarded as natural mortality. The cause of such mortality is seldom old age, but more commonly predation by other species or by members of the same population (cannibalism), disease, or injury due to a mechanical stress. The natural mortality of most marine organisms is very high when they are very young and vulnerable to predation.

The rate of natural mortality normally declines as individuals grow older. In fisheries management it is normally assumed that natural mortality is a constant proportion of the number of animals in the population, regardless of the size of the population. However, this proportion may differ between the different age classes in a population. As a general rule the basic assumption is that the larger the number of animals, the larger will be the number that die from natural causes. All mortality caused by fishing is classified as fishing mortality.

This includes fish which die as a result of fishing operations and which are not necessarily landed and recorded as part of the catch. One cannot simply estimate the total mortality in a population by assuming that natural mortality is independent of fishing mortality. Fish caught by fisherman may otherwise have been killed by other predators.

Therefore, fishermen can simply be viewed as additional predators competing with natural predators for a limited food supply. For example, a substantial amount of fish which are removed annually from the sea by fishermen are bound to die anyway, even if no fishing takes place. Therefore it is logical and correct to conclude that a high fishing mortality should be maintained when natural mortality is high, to avoid the loss of potential harvest to predators.

Growth rate

Like us, fish grow as they age. However, unlike us, most fish species grow throughout their lifetime, although the rate of growth declines with age. Aside from age, other determinants of growth rate for fish include gender, locality, water temperature, food availability and sea conditions. In the case of the South African west coast rock lobster, Jasus lalandii, females grow slower than their male counterparts after attaining sexual maturity. Conversely female hake of the species Merluccius capensis reach a larger size than their male counterparts.

Rock lobsters will not feed in rough sea conditions and rock lobsters living in shallow rough water will have a lower growth rate than those living under more protected water conditions. Shellfish such as the South African black mussel living in exposed and rough water conditions tend to spend a greater proportion of their energy on developing thick shells and byssus (hair like threads which keep the mussels attached to rocks in rough seas) than mussels living in more protected conditions.

Growth rate is also likely to be sensitive to population density because of changes in the intensity of competition for food, shelter and suitable substrate. While humans are unable to influence most factors affecting the growth rate of wild fish stocks, some factors are susceptible to human intervention, and this has a bearing on, for example, the type of fishing gear deployed and the proportion of the population harvested each year. The fact that most fish grow faster at lower population densities is a contributing factor to the need to thin out stocks in order to “stimulate” productivity. This and related issues will be discussed in future articles.

Another factor which should be considered is the possible effect of injury and/or shock suffered by fish which are caught and subsequently discarded (reasons could include size, wrong species, or lobsters in-berry) or which escape during the fishing operation. While there are no reliable statistics on the size of the effect of these factors on growth rate, it seems prudent to make allowance for a possible effect in the management process. In South Africa when a relatively large 89 mm minimum carapace size limit was in force for the West Coast rock lobster (Jasus lalandii) resource it was estimated that between 10 000 and 15 000 tons of undersized lobsters were discarded per annum in the late 1980s.

Many of these discarded lobsters suffered injuries, and a proportion probably suffered mortality. A decline in growth rate measured by mark recapture studies was ascribed to a shortage of food, but some scientists (the authors) theorised that the continuous selection for large lobsters removed fast growing individuals from the population, and this and the allocation of energy to the regeneration of broken limbs coupled with an impaired foraging ability might have caused the decline in growth rate. In the early 90’s it was unusual for lobsters to be caught with all their limbs intact.

In this and possibly other similar cases the impact of fishing on growth rate and mortality can be controlled to an extent and should be part of any harvesting plan.

Quantity versus quality in reproduction (r- and K- strategists)

All species groups can be crudely divided into those adopting one of two possible categories of reproductive strategy. The first is commonly known as an 'r-selected' strategy in which individuals produce very large numbers of offspring (many thousands or even millions per reproduction cycle) but invest very little energy in rearing them. Examples of species which fall within this category are marine shellfish like mussels, which can suddenly appear in enormous numbers on the coastline.

The second strategy is the so-called 'K-selected' strategy in which individuals produce few offspring at a time but invest large amounts of energy in rearing them. Examples include all marine mammals, for which the reproduction rate puts a ceiling on rate at which the population can grow. For example, the South African seal population is still recovering from sealing during the 19th century. The mortality rate of 'r-selected' offspring is very high (often 99% before reaching maturity) while 'K-selected' offspring have a much higher survival rate.

In general 'r-selected' organisms are short lived compared to 'K-selected' organisms (although some species become very long lived after crossing a certain size e.g. lobster) and while the first (r-selected) are better adapted to a dynamic and variable environment, the latter (K-selected) seem to be more resilient and better utilisers of a more stable and predictable environment.

Population biomass and its carrying capacity

The size of a population can be expressed either in terms of the number of individuals or in terms of biomass. Biomass is the mass of all living individuals in a population. It is used more commonly than population number, since catches are expressed in mass units (e.g. tons, kilograms). Population biomass is calculated from both the mass of individuals and their number in the population, and is a much more meaningful quantity for management purposes than number.

The population carrying capacity is the average size that an unexploited population reaches in its habitat under natural conditions. The carrying capacity is determined by a variety of biological and physical factors. For example, the carrying capacity for gannets along the South African west coast is probably determined by a combination of the amount of island breeding space, the amount of food, the number of predators, territorial behaviour, reproductive rate and strategy and susceptibility to disease.

Subsequent articles will start to develop these and other ideas in a quantitative framework to show their relevance to the actual management of marine fisheries.

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