Pelagic fish species assemblages in the southern Benguela

Abstract

Patterns in the co-occurrence of small pelagic fish species within single shoals were investigated using data from 6 814 throws of commercial purse-seiners in South Africa. Assuming that the throw composition reflected the true composition of the assemblage, it was shown that: (1) mixed pelagic assemblages were as prevalent as pure shoals; (2) assemblages of anchovy Engraulis encrasicolus and sardine Sardinops sagax exhibited a seasonal distribution pattern; (3) there was a highly skewed species ratio in terms of abundance by mass; and (4) patterns in the size distributions of two-species shoals were complex and dependent on the L_∞ and the relative abundance of the species concerned. We hypothesise that the observed patterns reflect the ‘net gain of the subordinate’, whereby fish occurring in small numbers are less conspicuous and/or less energetically attractive for potential predators if they are smaller than the dominant component of the school. If the subordinate fish grow larger than the dominant fish, this advantage persists. Potential sources of bias are alluded to but are not considered to have had a major impact on the conclusions reached, although they may form the basis for further work.

Keywords:

• Engraulis encrasicolus (= E. capensis)
• Etrumeus whiteheadi
• meeting point hypothesis
• mixed shoals
• oddity effect
• pelagic assemblages
• Sardinops sagax
• school trap hypothesis
• Trachurus capensis