Chokka squid (Loligo vulgaris reynaudii) abundance linked to changes in South Africa's Agulhas Bank ecosystem during spawning and the early life cycle

Abstract

Chokka squid biomass and catch are highly variable, likely owing to their links to changes in the ecosystem, which impact spawning and recruitment. A synthesis of basic ecosystem components for the domain in which chokka squid live (i.e. South Africa's west coast and Agulhas Bank) was prepared using published and new data. It included bottom temperature, bottom dissolved oxygen, chlorophyll, and copepod abundance. Alongshore gradients of these indicated that the main spawning grounds on the eastern Agulhas Bank are positioned where bottom temperature and bottom dissolved oxygen are optimal for embryonic development. This location, however, appears suboptimal for hatchlings because the copepod maximum (food for paralarvae) is typically on the central Agulhas Bank some 200 km to the west. Data on currents suggest that this constraint may be overcome by the existence of a net west-flowing shelf current on the eastern Agulhas Bank, improving survivorship of paralarvae by transporting them passively towards the copepod maximum. CTD data and a temporal analysis of AVHRR satellite imagery reveal the copepod maximum to be supported by a “cold ridge”, a mesoscale upwelling filament present during summer when squid spawning peaks. In situ sea surface temperature (SST) data used as a proxy for cold ridge activity demonstrate considerable interannual variability of the feature, especially during El Niño-Southern Oscillation events. Negative linear correlations between maximum summer SST (monthly average) and squid biomass the following autumn (r2 = 0.94), and annual catch (r2 = 0.69), support the link between the “cold ridge–copepod maximum” and the early life cycle of chokka squid, and holds promise for prediction.

Impacts of hypoxia on the structure and processes in pelagic communities (zooplankton, macro-invertebrates and fish)

“…Direct field observations show the sensitivity of some squid to low oxygen. Rabalais et al (2001) found squid in Remotely Operated Vehicle records only where DO was above 1.4 mL O 2 L −1 , and Roberts (2005) showed that Chokka squid (Loligo vulgaris) selects its spawning ground on the eastern Agulhas Bank where temperature and oxygen are optimum for the embryonic stages with ranges of temperatures between 11 and 20 • C and oxygen above 3 mL O 2 L −1 . Young stages seem to be more vulnerable to hypoxic conditions as shown by Hunt and Seibel (2000), who found juvenile squid Gonatus onyx at shallower depths with DO around 1 mL O 2 L −1 while the adults went deeper into the OMZ with DO less than 0.5 mL O 2 L −1 .…”

The São Paulo shelf (SE Brazil) as a nursery ground for Doryteuthis plei (Blainville, 1823) (Cephalopoda, Loliginidae) paralarvae: a Lagrangian particle-tracking Individual-Based Model approach

“…This is an important issue because recruitment strength variability in loliginid squid has been found to readily respond to interannual oceanographic anomalies (e.g. Roberts, 2005), some of which may not be reproducible in the hydrodynamic models if one uses averaged data.…”

“…This squid early life history-environment link is usually quite obvious and reflected directly in the fishery yields, as good or poor annual catches often follow years of good and bad environmental conditions, respectively (Vidal et al, 2002;Roberts, 2005;Roberts & Mullon, 2010). In other words, recruitment success or failure echoes the environmental conditions experienced by the paralarvae.…”

“…In other words, recruitment success or failure echoes the environmental conditions experienced by the paralarvae. As a result, both physical (temperature) and biological (paralarval abundance) environmental variables have been found to be reliable proxies for forecasting the recruitment (Sakurai et al, 2000;Agnew et al, 2002;Roberts, 2005;Zeidberg et al, 2006). Importantly, larval transport has been identified to be an essential ingredient for the recruitment, as current patterns can transport and retain the offspring in suitable or unsuitable environments for their post-hatching survival, growth and development (Martins, 2009).…”

The Use of Acoustic Telemetry in South African Squid Research (2003-2010)

“…As a result, the majority of research on the chokka squid has focused on inshore spawning, i.e. environmental effects on spawning (Augustyn, 1990, Roberts, 1998, 2005Roberts & Sauer, 1994;Roberts & van den Berg, 2002, 2005Sauer et al , 1992, the impact of fishing on spawning concentrations (Hanlon et al, 2002;Oosthuizen et al, 2002a;Sauer, 1995;Schön et al 2002), biological studies (Augustyn 1990;Lipinski & Underhill, 1995;Olyott et al, 2006;Roel et al, 2000;Sauer & Lipinski, 1990;Sauer, 1995;Sauer et al, 1992Sauer et al, , 1999, life cycle (Augustyn, 1990(Augustyn, , 1991Olyott et al 2007;Roberts & Sauer, 1994), feeding on the spawning grounds (Augustyn, 1990;Sauer & Lipinski, 1991;Sauer & Smale, 1991Sauer et al, 1992), spawning behaviour (Hanlon et al, 1994(Hanlon et al, , 2002Sauer, 1995;Sauer et al 1992Sauer et al , 1997Shaw & Sauer, 2004), the inshore spawning environment (Augustyn, 1990;Roberts, 1998Roberts & Sauer, 1994;Roberts and van den Berg, 2002;Sauer et al , 1992, the location of spawning grounds (Augustyn, 1990;…”