Comparing the coarse- and high-resolution horizontal and vertical egg distribution patterns of three clupeiform species in the southern Benguela ecosystem

Master Thesis


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University of Cape Town

Three fish species, anchovy Engraulis encrasicolus, sardine Sardinops sagax and round herring Etrumeus whiteheadi, co-exist over the western Agulhas Bank (W AB) in the southern Benguela upwelling ecosystem. To date, data on high-resolution distributions in ichthyoplankton in the region is lacking. To address this need, coarse and high-resolution ichthyoplankton data were collected using a California vertical egg tow (CalVET) net and a continuous, underway fish egg sampler (CUFES) for horizontal data, respectively, and plankton pump and a closing WP II net for vertical data. Simultaneous measurements of temperature were recorded during the CalVET net hauls. Eggs from vertically stratified samples were grouped into three categories based on their developmental stage using established protocols. Volumetric estimates of egg density (eggs.m-3) from on-station CUFES samples were significantly correlated with normalized areal (eggs.m-2) estimates of egg density from CalVET net samples at these stations for sardine (r 0.91), round herring (r=0.80) and anchovy (r = 0.73), demonstrating the validity of the CUFES sampling technique. Underway sample collection reduced the survey coefficient of variation for sardine and round herring eggs but not for anchovy eggs. Horizontal egg distribution patterns for anchovy showed that high egg densities were centered at two specific regions: midshelf (east of Cape Point) and offshore in the southern region of the study area, whereas those of sardines and round herring were more continuously distributed above and beyond the 200m isobath. Egg abundances for all species generally increased from inshore towards offshore. The mean egg densities for all three species were found to be spatially associated, whereas the station by station distribution patterns of anchovy eggs were spatially distinct from sardine and round herring. Eggs of all three species were concentrated near the surface (0-20 m), although proportions varied among species. All developmental stages of anchovy egg were confined close to the surface indicating that this species spawns near the surface. Sardine and round herring vertical egg profiles varied ontogenetically, even though all developmental stages were concentrated near the surface. A significant (> 50%) proportion of early egg stages were widely distributed between 0-60 and 0-80 m for sardine and round herring, respectively. These results therefore show that the spawning depths of these three c1upeiforms overlap to a large extent, but the intensity of spawning varied at depth for each species. In general the vertical thermal structure seemed to affect only how deeply the eggs are distributed rather than the mean egg abundance. The differences in vertical egg profiles among and within (with respect to ontogenetic variation) species were found to be statistically significant (Chi-squared test, P < 0.05). Investigations of spawning patterns in relation to temperature, longitude and latitude were used to compare the spawning habitats of the three species. Quotient rule analysis was used to distinguish the preferred temperature ranges of all three species. Round herring spawn over a wider temperature range (16.5-18.1°C) than anchovy (17.4-17.8° C and sardine (17-18.1 °C). Two spawning peaks were apparent for round herring (main peak at 17.2°C and secondary peak at 17.8°C), whereas only one peak at 18.0°C was apparent for anchovy and sardine. Anchovy spawning showed a domed-shape relationship with geographic location and the relationship was viewed to be more informative (r2 = 0.13 longitude, ~ = 0.11 latitude) compared to sardine (r2 = 0.08 longitude, r2 = 0.07 latitude) and round herring (r2 = 0.04 longitude, r2 = 0.06 latitude). These results indicate that anchovy were more specific in their selection of spawning habitat than sardine and round herring. Generally, these results suggest that the overall spawning habitats of anchovy, sardine and round herring on the W AB overlap to a large extent, but their exact locations of the point of spawning are spatially distinct.

Bibliography: leaves 122-159.