A recently published study has shed new light on the life cycle of deep-water hake Merluccius paradoxus, the species of Cape hake that accounts for approximately 85% of the catches of the deep-sea trawl fishery.
The study “Oxygen and temperature influence the distribution of deepwater Cape hake Merluccius paradoxus in the southern Benguela: a GAM analysis of a 10-year time-series”, was published in the African Journal of Marine Science. It was authored by SADSTIA research assistant, Fisokuhle Mbatha, and co-authors Dawit Yemane, Marek Ostrowski, Coleen Moloney and Marek Lipiński.
The study found that the environment plays an important role in regulating the distribution of different size classes of hake: the amount of dissolved oxygen in water close to seabed is the most important factor for small (≤15 cm) hake, whereas temperature is the most important variable for medium (16 cm to 34 cm) and large (≥35 cm) hake.
“The small M. paradoxus were associated with narrow ranges of oxygen concentrations and temperature. The medium M. paradoxus occurred in water with a narrow temperature range, having reduced occurrence at both the cool and warm ends of the measured range; they were also associated with moderate to high oxygen concentrations. The large hake occurred in water masses that were relatively cool and had elevated dissolved oxygen concentrations.”
This finding, when combined with other published information, suggests that the Orange Banks provide an important nursery ground for juvenile deep-water hake. The Orange Banks is an area of the continental shelf located near the international border between South Africa and Namibia (see map).
Although the Orange Banks has not yet been comprehensively studied, Mbatha et al. propose that juvenile deep-water hake take advantage of the relatively stable environmental conditions on the banks, where they find an abundance of suitable prey. Importantly, adults of both deep-water hake and shallow water hake are relatively scarce on the Orange Banks. This means that juvenile deep-water hake are safe from adult shallow water hake which are cannibalistic and routinely feed on juvenile deep-water hake.
Mbatha et al. suggest that the size and position of the nursery grounds for deep-water hake on the Orange Banks vary according to the area in which oxygen concentrations on the seabed are ideal.
Their study was informed by data collected over a period of 10 years from the research ship Dr Fridjof Nansen, which is operated by the Food and Agriculture Organization of the United Nations. The ship surveyed the waters of South Africa in 2003 and from 2005 to 2013.
The authors draw on the Dr Fridtjof Nansen survey data and knowledge of the lifecycle of deep-water hake published in four other studies. They confirm that the species spawns predominantly in South Africa, mainly on the western Agulhas Bank, from August to October.
The southwesterly winds that are so ubiquitous on the west coast during the summer months, play an important role because they drive the currents that transport hake larvae northwards towards the Orange Banks.
The study found that small deep-water hake generally occur on the continental shelf, in a band extending from Cape Point to the Orange River, while medium individuals displayed the widest distribution, reflecting their tolerance of a wider range of temperature and dissolved oxygen values. They occur on the continental shelf and at the shelf edge, whereas large deep-water hake are mostly found in deeper water, at the edge.
According to the authors of the study, distribution patterns are linked to the biology of the hake: small fish cannot survive in deep water because there are too many predators there – they gradually move into the deep as they approach adulthood because they themselves become efficient predators.
The study Oxygen and temperature influence the distribution of deepwater Cape hake Merluccius paradoxus in the southern Benguela: a GAM analysis of a 10-year time-series is available for download from the African Journal of Marine Science.