Life cycle of hake and likely management implications

Abstract

Despite its economic and social importance for Namibia and South Africa, limited documented information exists regarding key aspects of the biology of deep-water hake, including its life cycle. This study utilizes data collected through the demersal surveys of the R/V Dr Fridtjof Nansen in South Africa and F/V Blue Sea 1 in Namibia to describe the migratory patterns of deep-water hake in space and time. Furthermore the study investigates aspects of the life cycle of this important species in the Benguela region. Results show that deep-water hake spawns between the western Agulhas Bank and Elands Bay in South Africa with the main nursery ground between Hondeklip Bay and the northern tip of Orange Banks. Deep-water hake in Namibia (up to the Kunene River) and along the south coast of South Africa (eastwards to Port Alfred) originate from these grounds, and undertake long-range migrations across latitudes and longitudes, respectively. This hypothesis is supported by the finding that spawning has not been observed in Namibia and there are no small juveniles along the South African south coast from the eastern border of the Agulhas Bank. The proposed pattern implies an interconnection between the Namibian and the South African components of the stock and the consequent need for a revision of the present management regime based on the assumption of stocks confined within the respective national jurisdictions. This study has used length frequency distributions in space and time in order to investigate the life cycle, in terms of origin, movement and population structure in particular, an approach that may also be useful for other widely distributed species.

Effects of environmental variables on survey catch rates and distribution by size of shallow‐ and deep‐water Cape hakes, Merluccius capensis and Merluccius paradoxus off Namibia

“…Smallest size groups of M. capensis , that are significantly affected by oxygen levels, are found closest to the coast (also shown by Kainge et al., ), where hypoxic bottom waters are prominent. On the other hand, juvenile and larger M. paradoxus are mostly restricted to the southernmost part of the survey area, as also described by Strømme, Lipinski, and Kainge (), and both species occur deeper when getting larger. The hypoxic bottom waters are most extensive over the Namibian shelf during late austral summer to fall when the poleward advection of the warm, oxygen poor South Atlantic central water (SACW) from the Angola Gyre region onto the Namibian shelf is at its maximum (Duncombe Rae, ; Mohrholz, Bartholomae, van der Plas, & Lass, ; Monteiro et al., ).…”

Spatio‐temporal genetic structure and the effects of long‐term fishing in two partially sympatric offshore demersal fishes

“…Merluccius paradoxus appears to be expanding into northern Namibia and currently has higher abundance levels than M. capensis in this region (Stromme et al . ). Furthermore, the species may have adapted to a regime of fluctuating N e , buffering it against the effect of fishing.…”

“…Biological and ecological surveys suggest complex life histories for each species, with transboundary migrations for M. paradoxus (Stromme et al . ) and multiple spawning grounds for M. capensis reported across the Benguela region (Jansen et al . ; Wilhelm et al .…”

“…Distribution and abundance varies between taxa, with adult M. paradoxus considered a slope species, occurring mainly in deeper waters (200-800 m), and M. capensis considered a shelf species, with higher abundance at shallower depths (100-300 m; Payne & Punt 1995;Durholtz et al 2015). Therefore, the distribution of the two species is thought to be associated with local bathymetry and continental shelf width, with both moving to deeper waters as they mature (Jansen et al 2015;Stromme et al 2015;Wilhelm et al 2015). The Cape hakes are highly valuable commercial resources in the southern African region and support a multiuser fishery industry (DAFF 2014).…”

“…Biological and ecological surveys suggest complex life histories for each species, with transboundary migrations for M. paradoxus (Stromme et al 2015) and multiple spawning grounds for M. capensis reported across the Benguela region (Jansen et al 2015;Wilhelm et al 2015). The possibility of homing behaviour in both species has also been suggested based on fishery-independent data (Jansen et al 2015;Stromme et al 2015;Wilhelm et al 2015). Although these findings point to the occurrence of isolating mechanisms across the region, from a population genetic perspective the data are less clear.…”