Fisheries Biology, Assessment and Management

Michael King

• Publisher: Wiley-Blackwell
• Year: 2013
• ISBN:
  9781118688045
• Edition: 2
• Topic: Technology & Engineering
• Subtopic: Fisheries & Aquaculture
• Index: Technology & Engineering

This excellent second edition of Fisheries Biology, Assessment and Management, has been fully updated and expanded, providing a book which is an essential purchase for students and scientists studying, working or researching in fisheries and aquatic sciences.

In the same way that excessive hunting on land has threatened terrestrial species, excessive fishing in the sea has reduced stocks of marine species to dangerously low levels. In addition, the ecosystems that support coastal marine species are threatened by habitat destruction, development and pollution. Open access policies and subsidised fishing are placing seafood in danger of becoming a scarce and very expensive commodity for which there is an insatiable demand.

Positive trends include actions being taken to decrease the incidental catches of non-target species, consumer preferences for seafood from sustainable fisheries, and the establishment of no-take areas that provide refuges for marine species. But there is an urgent need to do more.

Because there is an increasing recognition of the need to manage ecosystems as well as fish stocks, this second edition of this bestselling text book includes an additional chapter on marine ecology. Chapters on parameter estimation and stock assessment now include step-by-step instructions on building computer spreadsheet models, including simulations with random variations that realistically emulate the vagaries of nature. Sections on ecosystem management, co-management, community-based management and marine protected areas have been expanded to match the increased interest in these areas.

Containing many worked examples, computer programs and numerous high quality illustrations, Fisheries Biology, Assessment and Management, second edition, is a comprehensive and essential text for students worldwide studying fisheries, fish biology, aquatic and biological sciences. As well as serving as a core text for students, the book is a superb reference for fisheries and aquatic researchers, scientists and managers across the globe, in both temperate and tropical regions. Libraries in all universities where fish biology, fisheries, aquatic sciences and biological sciences are studied and taught will need copies of this most useful new edition on their shelves.

Supplementary material is available at: www.blackwellpublishing.com/king

1

Ecology and ecosystems

1.1 Introduction

Studies in biology, the science of living things, can be directed at increasing levels of biological organization from molecules, cells, organs, to organisms (or species) and beyond to populations and ecosystems. This chapter is concerned with higher levels of biological organization – populations, communities and ecosystems. A population is a group of individuals belonging to the same species and a community is a collection of populations inhabiting a particular area. An ecosystem is a functional and relatively self-contained system that includes communities and their nonliving environment.

Studies in fisheries biology have been directed at particular species targeted by fishing operations. Now, particularly in the light of decreasing catches and threatened marine ecosystems, there is a need for fisheries managers to take a broader view, one that includes the interdependence of target species, other species and the marine environment. The ecosystem is the basic unit of ecology, and can be defined as the study of the interactions between groups of organisms and their environment. The environment of an organism includes all external entities and factors that affect it, and therefore includes physical factors such as light, temperature and oxygen as well as other living things such as competitors, mates, predators, and parasites.

Although it is common, and often useful, to apply the term ‘ecosystem’ to particular entities, such as coral reef ecosystems or estuarine ecosystems, it must be realized that these are not isolated units. Ecosystems are linked to one another by biological and physical processes. In marine ecosystems, these linking processes include biological factors, such as migration and food chains, as well as physical ones, such as ocean currents and tides. Pursuing these linking factors, it becomes apparent that the entire planet can be regarded as an ecosystem, and is sometimes referred to as a biosphere. However, a more restricted view of an ecosystem – as the plants, animals and environment of a particular type of habitat, such as a coral reef – provides a more manageable entity for study and management.

1.2 Distribution and abundance

Populations are groups of individuals belonging to the same species. In fisheries biology, the word ‘stock’ is sometimes used interchangeably (and loosely) with ‘population’. In the strict sense, a stock is a distinct, reproductively isolated population which exists within a defined spatial range.

1.2.1 Unit stocks

Fisheries studies and management are concerned with a unit stock, which may be defined as a discrete group of individuals that has the same gene pool, is self-perpetuating, and has little connection with adjacent groups of the same species. Although this definition may not satisfy biogeographers, it does describe a unit which, because it has similar biological characteristics, may be studied, assessed and managed as a discrete entity.

Some species exist within a wide geographic range as a collection of unit stocks. The cod, Gadus morhua, for example, is distributed across the North Atlantic (Fig. 1.1) and within this relatively large range, exists in more or less isolated subpopulations or races. In such cases, fishing on one subpopulation appears to have no effect on others.

Fig. 1.1 The distribution of the cod, Gadus morhua, in the North Atlantic.

The boundaries of a unit stock are often difficult to determine, and many seemingly isolated populations may receive new recruits from other distant reproducing populations. Even in stocks of fishes on isolated reefs, the ability of larvae to drift and survive for a considerable time in the plankton allows them to reach other reefs some distance away. For example, the snapper, Lutjanus kasmira, which was deliberately introduced into Hawaii, spread throughout all the reefs and islands of the archipelago within a period of ten years (Oda & Parrish, 1981). Non-migratory species that live in widely separated areas, such as seamounts, must either rely on larval drift to replenish their populations or, if their larval lifespan is short, be self-sustaining.

From a fisheries assessment and management viewpoint, it is important to determine whether two adjacent stocks are either sufficiently interactive to be regarded as a single unit stock, or independent enough to be treated as separate unit stocks. In most cases, several criteria are used to confirm or refute a stock’s unit status. The penaeid prawn, Penaeus latisulcatus, for example, is caught by trawlers in two adjacent gulfs in South Australia (Fig. 1.2) but not in the area of sea between the two gulfs. As each gulf contains mature individuals and has coastal mangrove areas where juveniles are found, the stock in each gulf has the ability for self-replenishment. In addition, as tagging studies have not revealed any migration of prawns between the two areas, the stocks in each of the two gulfs are regarded as two separate units for research and management purposes.

Fig. 1.2 Two separate unit stocks (shaded areas) of the penaeid prawn, Penaeus latisulcatus, in two gulfs in South Australia.

1.2.2 Spacing of organisms

Within a unit stock, individuals may be distributed uniformly, randomly or in aggregations (Fig. 1.3). A uniform or even distribution rarely occurs in nature, mainly because the environment is rarely uniform. Even if the environment is relatively even, such as on a sandy sea floor, the distribution of sedentary species is likely to be non-uniform as a result of the uneven settlement of larvae from the plankton. However, a uniform distribution may be approximated in species where there is competition, territoriality or aggression between individuals. Territorial reef fishes, for example, often exclude others of the same species from a range around a home base on the reef. Random distributions are also rare in nature, if only because the aspects of the environment on which the species depend, such as food and shelter, are not randomly distributed.

Fig. 1.3 Types of spacing of individuals within an area.

Although widely-spaced individuals avoid intraspecific competition, this is often at the expense of advantages which may accrue to those living in aggregations, groups, or schools, the most common type of distribution. The advantages of living in aggregations in mating species may include better access to sexual partners, and in broadcast spawners, an enhanced confluence of eggs and sperm. Aggregations may also provide an increased ability to locate food and a degree of protection from predators. For example, among aggregating sea urchins, fertilization success is high, the trapping of drift algae for food is enhanced, and the spine canopy of the aggregation is a formidable deterrent to predators. Whatever the spacing, the overall distribution of individuals or clumps will be influenced by differences or gradients in the environment. In all marine organisms, a differential distribution with depth is to be expected, and most species occur in maximum numbers over a relatively narrow optimal depth range.

In fisheries studies the estimation of abundance, or at least relative abundance (the number of individuals at one time relative to the number present at another time), is important in determining the effects of fishing and environmental disturbances. Methods of estimating abundance are presented in Section 4.2.

1.3 Population growth and regulation

Populations of all organisms fluctuate around a mean level as long as deaths are balanced by births. In cases of populations, such as many fish stocks, which are overexploited or threatened by environmental degradation, deaths will exceed births and numbers will decrease. When an exotic species is introduced into a ‘new’ and suitable environment (Section 1.5.4 ‘Species invasions, introductions and translocations’) its population will increase, often in the absence of predators, until it is contained by the lack of food or living space. This section provides an introduction to populations as well as the factors that affect and regulate them.

1.3.1 Population growth

In the absence of limiting factors in conditions of unlimited food and living space, the increase of numbers in a population would be immense. For example, if a female shrimp produced 50 thousand female larvae, her descendants could number 2500 million females after just one additional annual spawning event as long as all the resulting larvae survived to reproduce. If N is the number of individuals in the population at a particular time, b is the birth rate, and d is the death rate, the population growth rate (I) is:

(1.1)

It is the value of (b − d), referred to as the intrinsic rate of population increase, that determines whether a population will decrease, remain stable (at zero population growth) or increase. As long as the average birth rate, b, exceeds the average death rate, d, the population (N) will increase. In addition, if populations are increasing, N will become larger with each generation causing the rate of increase to rise further. This multiplying rate of increase causes population numbers to increase as shown in the left-hand curve in Fig. 1.4: the curve becomes steeper and steeper until the population is expanding at an infinite rate. All organisms have such potential for exponential growth in the absence of any limiting environmental factors. But, as the world is not packed full of shrimps or any other organism, it is obvious that the increase in numbers in real populations is being held in check, or regulated, by one or more factors.

Fig. 1.4 Population growth curves, (from the left) without limits, with density-dependent limits and with density-independent limits.

1.3.2 Population regulation

All populations are limited in abundance by their requirement for resources – for essentials such as food and living space. Competition for these resources, and predation, cause the rate of population growth to decrease at high densities. These limiting factors are regarded as density-dependent because their effects increase as population density increases – for example, the effects of shortages of food and living space (starvation and crowding) increase with population size. Over time, population numbers follow an S-shaped curve in which an initial increasing rate of growth is followed by a decreasing rate as the curve approaches an asymptote (at what is known as the carrying capacity of the environment) imposed by one or more limiting factors (middle curve in Fig. 1.4).

Although populations of all organisms have limits to their growth, the human population is increasing, seemingly without limit. The carrying capacity of the planet in relation to human food has been increased by highly productive agricultural systems (although the distribution of food is inequitable) and more stringent social controls have allowed people to live in more crowded living spaces. How long this high rate of human increase can continue is not known. But leaving quality of life and aesthetic considerations aside, each additional person contributes to the planet’s environmental ills and the demand for natural resources. The overexploitation of fisheries resources and degradation of the environment are attributable to the rapidly increasing human population.

The numbers in some populations vary around the carrying capacity of the environment but in others, numbers fluctuate widely in response to factors that are unrelated to population density. The classic terrestrial example of this is the fluctuating population of some insects that appear in large numbers in warm humid conditions and disappear as the weather becomes cooler and drier. In this case, the weather is imposing a density-independent limitation on population size – that is, the weather is unrelated to (is independent of) population density.

Some species of shrimps or prawns, often regarded as the insects of the sea, also have populations that vary greatly from year to year depending on conditions in shallow nursery areas. Populations of some species of fish thrive in brackish-water estuaries during ideal conditions but die in large numbers with influxes of freshwater during heavy rain and floods. Populations of molluscs living in shallow tidal pools may suffer high mortalities during extended periods of low tides and hot weather when water temperatures climb and the amount of dissolved oxygen decreases. Storms and human interference, including shoreline building projects, produce silt that reduces the amount of sunlight reaching light-dependent organisms, such as algae, giant clams and coral. In these examples, salinity, temperature, dissolved oxygen and subsurface light are imposing density-independent limits on populations.

1.3.3 Life history patterns

The evolution of a particular life-history pattern in a species, including specific growth rates, mortality rates, and reproductive strategies, depends on a complex array of selective forces imposed on a species by its environment. Stock numbers are a result of the recruitment rate, birth rate and mortality rate of the species and are under the control of density-dependent and densityindependent effects.

In stable or predictable environments, species are more likely to be under the control of densitydependent effects, such as competition for food and space, and stock sizes may be relatively constant over time. In more variable environments, density-independent effects, such as extreme water temperatures, storms, and adverse currents, are likely to result in stock numbers fluctuating over time;...