Small pelagic fish responses to fine-scale oceanographic conditions: implications for the endangered African penguin

Abstract

International audienceSmall pelagic fish play a significant role in regulating the foraging activities and population trends of marine top predators in upwelling ecosystems, yet there is little information on oceanographic drivers of fish assemblages at temporal and spatial scales relevant to their predators. The survival of the Endangered African penguin Spheniscus demersus is closely linked to the availability of pelagic fish prey. This study assesses the influence of oceanographic variables on the spatio-temporal dynamics of pelagic fish in Algoa Bay, South Africa, where half of the world population of African penguins breed on 2 islands, St Croix and Bird. Using small-scale acoustic surveys and an array of underwater oceanographic data recorders spread across the bay during 3 yr, we reveal the complex and variable nature of this system, with fish responding differently to physical processes around each island. Chlorophyll a concentrations were good predictors of relative fish abundance around Bird Island but had little influence around St Croix Island, possibly due to the masking effect of purse-seine fishing around this site. The horizontal distribution of fish schools around Bird Island was more aggregated under upwelling conditions (cooler sea surface temperatures) and the vertical distribution of fish around both islands was strongly influenced by stratification and mean temperatures. Mechanistic drivers of upwelling included northeasterly winds and offshore Natal pulses, both of which are predicted to have an increasingly more significant effect on the suitability of habitat for pelagic fish and associated predators under the influences of climate change and industrial fishing

Influence of environmental variables on the larval stages of anchovy, Engraulis encrasicolus, and sardine, Sardinops sagax, in Algoa Bay, South Africa

“…On the other hand, strong northeasterly winds in the region promoteupwellings (Schumann et al 1991). The upwelling events associated with easterly winds occurred mainly in the post-drought years (McInnes et al 2017). This supports the results of our GAMs, which show a significant influence of high levels of easterly wind speed and Chl a concentration on anchovy larvae.…”

“…Possibly, this effect has been counterbalanced by the moderate rains in the post-drought period, but especially by the relatively strong easterly winds. Moreover, McInnes et al (2017) described an anusual spike in Chl a in Algoa Bay during April-May 2012 and again in May 2013, and they linked it to water pulses offshore in the Agulhas Current and the associated influxes of cool, upwelled waters into the Bay. McInnes et al (2017) claimed that, through an enhancement in the regional primary production, these pulses led to the increase in pelagic fish abundance, especially around Bird Island.…”

Targeted prey fields determine foraging effort thresholds of a marine diver: Important cues for the sustainable management of fisheries

“…Cairns & Schneider, ; Gorke & Brandl, ), including penguins (Baylis et al, ), and the foraging locations of diving seabird species are known to be correlated with favourable depth distributions of their prey (Boyd et al, , ; Zamon et al, ). Foraging site selection by African penguins in Algoa Bay, especially for penguins breeding on Bird Island, may be related to the dynamic nature of oceanographic processes in this region (van Eeden, Reid, Ryan, & Pichegru, ; Goschen, Schumann, Bernard, Bailey, & Deyzel, ) and the influence that these processes have on the three‐dimensional distribution of small pelagic fish around Bird and St Croix islands (McInnes et al, ). Foraging locations east of Bird Island are close to upwelling areas (Goschen et al, ) which experience elevated levels of chlorophyll a (chl a ) that are positively associated with pelagic fish abundance (McInnes et al, ).…”

“…Foraging site selection by African penguins in Algoa Bay, especially for penguins breeding on Bird Island, may be related to the dynamic nature of oceanographic processes in this region (van Eeden, Reid, Ryan, & Pichegru, ; Goschen, Schumann, Bernard, Bailey, & Deyzel, ) and the influence that these processes have on the three‐dimensional distribution of small pelagic fish around Bird and St Croix islands (McInnes et al, ). Foraging locations east of Bird Island are close to upwelling areas (Goschen et al, ) which experience elevated levels of chlorophyll a (chl a ) that are positively associated with pelagic fish abundance (McInnes et al, ). The increase in penguin foraging effort during 2012 coincided with reduced and more erratic chl a and associated pelagic fish abundance estimates with the lowest and highest prey abundance recorded during this study for the months of April and May respectively.…”

“…Echo‐returns were processed using Fish Image Segmentation Helper (FISH) software following the procedures outlined in McInnes et al (); see Appendix in Supporting Information for details. Two metrics of relative fish abundance were calculated: total volumetric abundance (TVA, kg) for each fish school (see McInnes et al, for details), and the nautical area scattering coefficient (s A , m 2 nmi −2 ) aggregated over the survey length.…”