Use of environmental parameters to explain the variability in spawnerrecruitment relationships of Namibian sardineSardinops sagax

Abstract

This study attempts to explain the variability in recruitment of sardine in the northern Benguela and to develop potential models by including environmental information to predict recruitment. Two different recruitment and spawner number datasets were available: a VPA-developed dataset, for the period 1952-1987, and data from a simple age-structured model for 1992-2007. In all, four environmental indices were used: the degree of the intrusion of the warm Angola Current into northern Namibia, termed the Angola-Benguela front index; the extent of the upwelling area off central Namibia; average sea surface temperature (SST) over the northern and central Namibian shelf; and wind stress anomalies at Lüderitz as an indicator of upwelling strength. Contrary to general belief, it was found that extremely high recruitment can happen at low spawner levels. This occurred in years in which a large upwelling area existed in association with the minimum southward intrusion of the Angola Current. These effects override the normal negative linear relationships with SST and the positive linear relationship with wind. However, when the area of upwelling is average or small, the effects of spawner biomass, SST and wind become important factors in the variability of recruitment. To estimate exceptional recruitment, the upwelling and front indices were included in the model. To measure medium and weak recruitment, spawner numbers and the SST and wind anomaly formed part of the model. These models can be used simultaneously to predict recruitment before annual acoustic surveys take place and thus aid management decisions.

The selection of spawning location of sardine (Sardinops sagax) in the northern Benguela after changes in stock structure and environmental conditions

“…, 1987), which creates a relatively sheltered area conducive to near‐surface thermal stratification (le Clus, 1990). During the early 1980s, the preferred spawning locations of sardine were around 19°30′S, where the shelf is narrower and conditions are much more variable due to the close proximity to the Angola–Benguela frontal zone and the strong upwelling cell off Cape Frio (19°S) (Kirchner et al. , 2009).…”

“…Here the coastline follows a slightly concave orientation and the shelf is fairly wide (Boyd et al, 1987), which creates a relatively sheltered area conducive to nearsurface thermal stratification (le Clus, 1990). During the early 1980s, the preferred spawning locations of sardine were around 19°30¢S, where the shelf is narrower and conditions are much more variable due to the close proximity to the Angola-Benguela frontal zone and the strong upwelling cell off Cape Frio (19°S) (Kirchner et al, 2009). The shift in spawning location with regard to longitude from around 12°to 13°45¢E can be explained by the slight northwest orientation of the Namibian coast.…”

“… Biomass and catches of sardine in northern Benguela from 1947 to 2008 (adapted from Kirchner et al. , 2009).…”

Assessment and management of Cape horse mackerelTrachurus capensisoff Namibia based on a fleet-disaggregated age-structured production model

“…low to high SSTs and hence decreased upwelling since the late 1980s and early 1990s (Bartholomae and van der Plas 2007). Also, the reduction or near absence of other pelagic species (sardine) much sought by top predators to a dominance by other species, including juvenile horse mackerel Roux 2006, Mecenero et al 2007), would have a negative effect on the survival of horse mackerel until recruitment into the fishery, may be another reason why the productivity of horse mackerel could have declined since the collapse of Namibian sardine since the early 1980s (Crawford et al 1987) and again in mid-1990s , De Oliveira et al 2007, Kirchner et al 2009). …”

Stock-environment recruitment analysis for Namibian Cape hake Merluccius capensis

“…The northern Benguela current ecosystem off the central and northern Namibian coast has a pronounced influence on fish stock variability (Voges et al 2002, Bartholomae and van der Plas 2007, Kirchner et al 2009). Coastal upwelling is the most influential oceanographic feature of this region (Boyd and Agenbag 1985) determining, to a large extent, the seasonality and variability of sea surface temperature (SST) (Gordoa et al 2000).…”