Otolith δ18O and microstructure analyses provide further evidence of population structure in sardine Sardinops sagax around South Africa

Abstract

Sardine Sardinops sagax is an ecologically and economically important Clupeid found off the entire South African coast that includes both coastal upwelling and western boundary current systems. Although the management of the sardine fisheries historically assumed a single, panmictic population, the existence of three, semi-discrete subpopulations has recently been hypothesized. We conducted otolith δ18O and microstructure analyses to investigate nursery habitat temperatures and early life growth rates, respectively, of sardine collected from three biogeographic regions around South Africa’s coast to test that hypothesis. Analyses indicated that for both summer- and winter-captured adults and summer-captured juveniles, fishes from the west coast grew significantly slower in water that was several degrees cooler than those from the south and east coasts. This suggests that mixing of sardines between regions, particularly the west and other coasts, is relatively limited and supports the hypothesis of semi-discrete subpopulations. However, the west-south differences disappeared in the results for winter-captured juveniles, suggesting that differences in early life conditions between regions may change seasonally, and/or that all or most winter-captured juveniles originated from the west coast. Further elucidating the interactions between South African sardine subpopulations and the mechanisms thereof is important for sustainable harvesting of this species.

Contrasting life-history responses to climate variability in eastern and western North Pacific sardine populations

“…The differences in temperature and M oto histories and otolith growth trajectories suggest JP sardine has a warmer nursery environment, higher metabolic rate, and faster growth rate than CA sardine. These are consistent with the differences between populations of the western and eastern boundary current systems that support sardine near South Africa 30 , likely reflecting the differing ocean environments in the western (warmer) and eastern (cooler) boundary current systems.

Fig.

…”

“…2a ). Despite the vast geographic distances that separate sardine populations in South Africa from those in the North Pacific, growth histories of otolith radii of JP sardine and sardine from the south-east coast of South Africa 30 were similar until 100 dph. Likewise, the growth histories of CA sardine and sardine from the west coast of South Africa 30 were similar until 100 dph (Fig.…”

“…A recent study of sardine Sardinpos sagax ocellatus off South Africa, which includes both a western and an eastern boundary current system (Fig. 1b ), showed that the sardine subpopulation off the south and east coasts (the warm, western boundary current system) grew faster under approximately 3 °C warmer water than that off the west coast (the cool, eastern boundary current system) 30 . As the differences follow the distinct oceanographic conditions of the western and eastern boundary current systems, they may be the common features of populations in subtropical boundary current systems around the globe.…”

The sardine run in southeastern Africa is a mass migration into an ecological trap

“…Our study provides new insights into the causes of the sardine run. First, it provides the first molecular support for a previously proposed two-stock hypothesis based on nongenetic data (8,(10)(11)(12)(13)(14)(15)(16)(17)(18). This suggests that the previous evidence for stocks following the definition of Ihssen et al (35) as being "intraspecific groups of randomly mating individuals with temporal and spatial integrity" is not merely a result of phenotypic plasticity in regional assemblages affected by different environmental conditions but that it has a heritable basis.…”

“…Sardines from these two South African regions spawn at different temperatures (10) and have different nursery areas (11,12). These and other differences (13) suggest that they comprise distinct western and southern stocks (8,(14)(15)(16)(17)(18), but genetic studies have so far failed to confirm this (19).…”