Intensive fishing and a ‘keystone’ predator species: Ingredients for community instability

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  • Biological Conservation 22 (1982) 273 281

    INTENSIVE F ISHING AND A 'KEYSTONE' PREDATOR SPECIES: INGREDIENTS FOR COMMUNITY

    INSTABIL ITY

    GERALD B. GOEDEN

    Senior Fisheries Biologist, Queensland Fisheries Service, Northern Fisheries Research Centre, Bungalow, Queensland 4870, Australia

    ABSTRACT

    A series of surveys ofdemersalfish stocks were carried out along the Great Barrier Reef in order to assess their condition and to investigate the effects of fishing on the large-predator community. Results from the surveys indicate that a significant relationship exists between the abundance of a 'keystone' predator species and the distance from major human population centres. Removal of this predator through .fishing has the effect of bringing about changes in the relative abundance of other large predatory species including several that do not comprise part of the usual catch.

    Although the changes in community structure are apparently unpredictable, it is possible to calculate a measure of community flux which reflects the rate of this change relative to variations in the abundance of the keystone species.

    INTRODUCTION

    Demersal reef fish surveys carried out between 1977 and 1980 were directed at establishing the extent and condition of commercial and recreational species stocks along the Great Barrier Reef. These surveys were carried out in response to an increasing belief among fishermen that the abundances of certain desirable species were declining over much of the fishing grounds.

    Because there is a tendency for commercial and amateur fishing groups to blame each other for a reduction in catches (Goeden, 1975; Ruello & Henry, 1977), the surveys were planned for areas ranging from entirely commercial to entirely recreational and sampling included some species which are rarely taken by either party.

    273 Biol. Conserv. 0006-3207/82/0022-0273/$02.75 Applied Science Publishers Ltd, England, 1982 Printed in Great Britain

  • 274 GERALD B. GOEDEN

    METHODS

    Representatives of four families ofdemersal fishes were selected for visual surveys of population densities on 76 coral reefs. Visual surveys of other demersal fishes have been accomplished by the observer swimming along a measured transect (Brock, 1954; Russell, 1977).

    Our surveys were carried out by a team of two (one boat operator and one observer) from a 4-m outboard-powered boat. The operator maintained a towing speed of about 2 knots and kept the boat in water ranging between 3 m and 15 m deep. The observer followed the reef contours underwater with a manta-board while maintaining an altitude above the bottom of about 3 m. The length of each sample was measured with a propellor-driven counter mounted on the manta-board. The observer recorded the number of individuals of selected species along a 5 m wide track (indicated by brackets on the manta-board).

    These data, together with the total area covered, were used for the computation of population density indices which are estimates of the actual population densities of particular species.

    Table 1 gives the most commonly encountered species amongst the large, conspicuous demersal predators and comprising the four groups of reef fishes selected for these samples.

    The reef fish community was sampled in 8 general areas (Fig. 1) from Melville Passage south to the Capricorn Group. Table 2 describes the sampling within each general area.

    Tests for the difference between fish populations at each sample site combination were carried out using the Kolmogorov-Smirnov two sample test (Siegel, 1956). Where large numbers of samples were obtained, the data from a single, smaller reef were pooled and the entire reef was treated as a single sample. Correlation was tested using the Spearman rank correlation coefficient, r s.

    The Heron-Wistari demersal reef fish community received only light fishing pressure during the five years prior to and during the survey period as the result of two closures of the grounds and the active discouragement of fishing by the local resort. The Heron-Wistari community provides a base-line measure for an area with only minimal fishing. Such an area is considered to be 'undisturbed' for comparative purposes within this work.

    The relative dominance of individual species, in terms of biomass, within the lightly fished Heron-Wistari community was established by weighting the relative abundance of each species with the average total length (estimated to the nearest 5 cm) of the first 50 individuals recorded within the transect. Plectorhyncus schotaf was not recorded at Heron-Wistari. Its average length was estimated at Wheeler reef where the population density was greater. Specimens of each species of total lengths similar (nearest 5 cm) to that of the computed average total lengths were obtained, weighed, and used to calculate an estimate of average biomass.

  • EFFECT OF FISHING ON PREDATOR SPECIES

    TABLE 1 LIST OF THE MOST COMMONLY ENCOUNTERED SPECIES OF THE LARGE, CONSPICUOUS

    DEMERSAL PREDATORS

    275

    Species Common name

    Plectorhynchidae Plectorhyncus chaetodontoides (Lacepede) P. goldmanni (Bleeker) P. schotaf (Forskal) Spilotichthys pictus (Thunberg)

    Lethrinidae Lethrinus chrysostomus (Richardson) L. nebulosus (Forskal)

    Plectropomidae (after Smith, 1949) Plectropomus leopardus (Lacepede) P. melanoleucus (Lacepede) Plectropomus sp. (apparently undescribed)

    Serranidae Anyperodon leucogrammicus (Valenciennes) Cromileptes ahivelis (Valenciennes) Epinephelus fasciatus (Forskal) E. megachir (Richardson) E. merra (Block) E. tauvina (Forskal) Epinephelus spp. Variola louti (Forskal)

    Many-spotted sweetlip Many-lined sweetlip Grey sweetlip Painted sweetlip

    Red-throated emperor Spangled emperor

    Coral trout Tiger trout Oceanic trout

    White-lined rock-cod Barramundi cod Black-tipped rock-cod Long-finned rock-cod Honeycomb rock-cod Estuary rock-cod

    Lunar-tailed rock-cod

    i ,

    MELVILLE PASSAGE

    Cairns

    T0wnsvitte

    .~.. CAIRNS ~{ I.-~INNISFAIL

    ~ ~ '....~,., WHEELER

    "~" .:.

    Queens land

    Rockha~

    SWAIN !:-:.'~..[ REEFS

    ~--~--CA~RIEORNS

    1/,&E 150E 15&fie

    Fig. 1. The general location of surveyed reef areas along the Great Barrier Reef, Queensland, Australia.

  • 276 GERALD B. GOEDEN

    TABLE 2 DESCRIPTION OF THE AREAS OF SAMPLING ALONG THE GREAT BARRIER REEF.

    Area name Date Number Total Number of sample area of reefs

    samples (ha) sampled

    Swains December 1976 151 94.2 30 October 1977

    Capricorns December 1977 18 10.5 3 Heron-Wistari December 1977 10 6.9 2 Wheeler September 1977 5 6.2 1 Innisfail November 1977 22 22.0 3 Cairns February-May 1978 34 48.1 6 Melville May 1978 20 10.0 3 Cooktown January 1980 96 79.7 28

    Total 356 277.6 76

    RESULTS

    The ranked population density indices for plectropomids at each reef area followed the descending order of Heron-Wistari, Swains, Melville Passage, Wheeler, Capricorns, Innisfail-Cooktown, and Cairns. The Cooktown samples covered such an extensive area (more than 6000 km 2) and included such a variety of reef types that they cannot be readily compared with the other reef areas.

    Heron-Wistari was significantly different from all others areas (p < 0-05). The remaining areas within the hierarchy were different when separated by one or more ranks. Adjacent ranks (excepting Heron-Wistari) were similar i.e. p > 0.05.

    There are now a large number of small boats of limited range associated with the amateur or recreational fishery. Increased fishing pressure resulting from the addition of their numbers to the fishing 'fleet' appears to have been primarily directed toward those reef areas that are most accessible. Figure 2 shows the significant relationship (r s = 0-703, p < 0.0005) between the plectropomid popu- lation density indices for the 40 reefs between Cairns and Melville Passage and their distances from Cairns. The two reefs signified by squares are popular anchorages for game fishing and charter boats.

    On a larger scale is the comparison of the plectropomid population density indices for entire reef areas along the Great Barrier Reef with their respective distances from major human population centres. This relationship, expressed as p = 0.05D + 5.7 where P is the population density index and D is the distance from major centres in km, is significant (r s = 0.929, p < 0.05), and provides a predictive model for the abundance of plectropomids along the Great Barrier Reef.

    Extrapolation to the Heron-Wistari plectropomid population density index (40.9 fish ha- 1) indicates that undisturbed populations could only exist more than 700 km from major centres. No large reef areas fall into this category and it appears

  • Fig. 2.

    EFFECT OF FISHING ON PREDATOR SPECIES 277

    2,

    - 21

    X

    ~ 12

    N 9

    6

    3

    o 16o 260 36o ~,6o DISTANCE FROM CAIRNS {KM)

    Plectropomid population density indices related to distance from the nearest major city for reefs between Cairns and Melville Passage.

    likely that there are no large areas of the Great Barrier Reef that now support plectropomid stocks in an unexploited condition.

    Because the plectropomids represent about 30 ~o of the demersal reef fish catch landed at the Queensland Fish Board and for the spearfishermen are 'the most commonly-landed highly prized food fish in Queensland waters' (Saenger, 1978), some measure of its significance within the demersal predator community was sought. The measure used was the estimated percentage of the selected predator community biomass (Table 3).

    It can be seen that P. leopardus