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

Abstract

In order to study the effects of temperature, oxygen, salinity and time of day on survey trawl catches, we modeled observed catches of juvenile, small, medium and large hakes per station as functions of zenith angle of the sun, geographical position, year, temperature, salinity, oxygen and depth. We used data from summer demersal surveys conducted during the period 2002–2015, together with a computation of the corresponding light level data from which the solar zenith angles were obtained, and fitted the generalized additive models to these data. Based on best model results, important covariates were oxygen, depth, geographical position and temperature. The best models explained 70%, 69%, 57% and 57% of the variability in catches of juvenile, small, medium and large Merluccius capensis, respectively, and 71%, 68%, 81% and 70% of juvenile, small, medium and large Merluccius paradoxus, respectively. The significant effects of temperature, oxygen, depth and geographical position on survey catches of hake of different size groups indicate that survey size structure may be affected by the behavior of both species towards environmental conditions. Greater care should therefore be taken when interpreting hake survey biomass estimates, based on swept area method, especially those that were collected during exceptional unfavourable environmental conditions. It would also be highly desirable if the oceanographic conditions are collected on each trawl station in order to improve understanding of the linkage between resources and environmental conditions.

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

“…Small M. paradoxus were associated with environmental oxygen ranges that are restricted, indicating that their presence was likely associated with hypoxic conditions (<2 ml O 2 l -1 ) (Monteiro and van der Plas 2006), although the small fish mostly inhabited oxygen-depleted water (2-3 ml O 2 l -1 ) and appeared to occur less often in waters with high oxygen concentrations (>3 ml O 2 l -1 ). In an analysis of the catch rates of both hake species, Kainge et al (2017) found that oxygen was not an important covariate. Singh and Lamont (2016) found that M. paradoxus juveniles mostly occurred in moderately oxygenated water (2-3.5 ml O 2 l -1 ).…”

“…Singh and Lamont (2016) mostly found M. paradoxus juveniles in moderately oxygenated waters (2-3.5 ml O 2 l -1 ) and it appeared that temperature was more important than oxygen in determining the distributions of M. paradoxus juveniles. Kainge et al (2017) found that juveniles of M. capensis were more sensitive than juveniles of M. paradoxus to dissolved oxygen concentrations.…”

“…Olivar et al (1988) indicated that temperature and oxygen have strong influences on hake distribution and abundance. In a specific finding of Kainge et al (2017), it was assumed that, based on the model they used, M. paradoxus are mainly influenced by temperature and geographical position.…”

Temperature, Salinity and Oxygen Climatology of African Continental Shelf Water, South of the Equator, and Changes Since 1945

“…• Decreased pelagic fish catches off Angola from the late 1970's to 1990's, with recoveries since the mid-2000's (Kainge et al, 2020). Oxygen has been shown to be a key determinant of species such as juvenile deepwater hake in the Northern Benguela (Kainge et al, 2017) and the climatologies document warmer temperature and higher oxygen levels, coinciding with the recovery of fisheries.…”

“…The African continental shelf is host to natural resources of particular and critical socio-economic value for coastal communities (Kainge et al, 2020). Understanding the significance of observed changes in ocean variables such as temperature and wind (Roualt et al, 2010;Leduc et al, 2010;Santos et al, 2012;Beal et al, 2016;Vizy et al, 2018;Malan et al, 2019), and ecosystem changes such as species range shifts and variability in species abundance (Cockcroft et al, 2008;Roy et al, 2007;Coetzee et al, 2008;Yemane et al, 2014;Blamey et al, 2015;Jarre et al, 2015a;Van der Linge et al, 2016;Kainge et al, 2017;Van der Linge and Hampton, 2018), have transdisciplinary significance (Potts et al, 2014;Augustyn et al, 2017;Hobday and Pecl, 2014;Kainge et al, 2020). Construction of climatological maps of key ocean variables, based on the calculation of mean fields from historical data sets, are valuable reference tools in this regard.…”