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.

Towards an ultimate explanation for mixed‐species shoaling

“…Mixed-species shoaling behaviour has been described across a range of aquatic systems (Appendix S1), in some cases being as common or more common than single-species shoaling (e.g. temperate lake fishes [Krause, Godin, & Brown, 1998], and small pelagic fishes [Louw, Fréon, Huse, Lipiński, & Coetzee, 2014]), however, inherently contradicts the view that similarity in phenotype, behaviour and physiology among shoal members is a central contributor to fitness. Unsurprisingly, the body of research from which our understanding of shoaling behaviour is derived has historically centred on singlespecies shoals (Breder, 1959;Pitcher, 1993;Shaw, 1978;Sridhar & Guttal, 2018), with a relatively small number of publications invested in understanding the significance of mixed-species shoaling ( Figure 1).…”

Assessing the South African sardine resource: two stocks rather than one?

“…Both anchovy and round herring have an associated sardine bycatch (Louw et al 2014). The juvenile sardine bycatch with anchovy is non-negligible, increasing with the size of anchovy catch allowed, and consequently impacts the size of the directed adult sardine catches that can be permitted in subsequent years.…”