Comparison of fish-community size spectra based on length frequencies and mean lengths: a note

  • D Yemane Marine Biology Research Institute, Department of Zoology, University of Cape Town, Private Bag, Rondebosch 7701, South Africa
  • M H Griffiths Formerly Marine and Coastal Management, Department of Environmental Affairs and Tourism, Private Bag X2, Rogge Bay 8012, South Africa; now Ministry of Fisheries, PO Box 1020, Wellington, New Zealand
  • J G Field Marine Biology Research Institute, Department of Zoology, University of Cape Town, Private Bag, Rondebosch 7701, South Africa
Keywords: comparisons, length frequency, mean length, size spectra

Abstract

Estimates of fish-community size spectra are promising indicators of the impact of fishing on fish assemblages. Size spectra consist of logarithmic graphs of abundance plotted against fish body size. Size spectra may either be constructed from length frequency data or estimated from the mean sizes and abundances of the species in an assemblage — very often mean sizes are the only historical data available. Changes in the slopes of size spectra are interpreted to indicate changes in the relative abundances of small vs large fish, whereas changes in intercept (height) suggest changes in the overall abundance of the fish assemblage. A comparison of the size spectra of linefish catches of the Cape region, South Africa, for the period 1986–1998 reveals that statistics of the size spectra calculated from mean length data are significantly larger (heights) and shallower (slopes) than those calculated using length frequencies (paired t-tests, p < 0.001). Therefore, use of mean lengths in size spectra overestimates the overall abundance of the community. Mean length was also found to overestimate the relative abundance of larger fish in the assemblage, thereby underestimating the effects of fishing. In a time-series of size spectra, it is therefore necessary to use one method consistently for comparative purposes.

African Journal of Marine Science 2005, 27(1): 337–341
Published
2005-06-30
Section
Articles

Journal Identifiers


eISSN: 1814-2338
print ISSN: 1814-232X