The early life strategy of Cape hakes in the Benguela upwelling system off South Africa

Abstract

Recruitment is a key process in the sustainability of marine fish populations. Thedynamic and complex process of recruitment is integrated over several life stages, withmany different factors affecting each stage leading to high variability in populations. Thisstudy aimed at gaining important knowledge on the early life history and the factorsaffecting recruitment of the two demersal Cape hake species Merluccius paradoxus andM. capensis in the southern Benguela upwelling system off South Africa. Eggs andlarvae were collected by stratified hauls during four cruises in two consecutive years.Early juveniles were collected on one cruise in the northern part of the study area off theOrange River.The spatial distribution patterns, growth and condition of early stages were investigatedusing a combination of classic and novel methods. Spatial distribution patterns weredescribed by species-specific vertical and horizontal abundances. Growth wasinvestigated by otolith analyses of larvae and juveniles. Larval conditions were analysedby measuring RNA:DNA ratios. Another method for estimating condition was applied toeggs and larvae by measuring the total lipid content. Fatty acid compositions wereinvestigated in order to reveal the levels of essential fatty acids, needed for growth anddevelopment. Stomach content analyses complemented the information on early lifestages as they elucidated the feeding of larvae in the study area.Distribution patterns revealed that M. paradoxus was the dominant species during thestudy periods, indicating a temporal and possibly spatial separation of spawning of thetwo Cape hake species. This difference in the spawning strategy reduces competitionduring the early life stages between the two similar species, allowing their co-existencewithin the same ecosystem.Furthermore, the distribution patterns of hake eggs and larvae indicated that asubstantial part of spawning occurs on the western Agulhas Bank. Early stages aretransported by the jet current, whereby the drift routes of the two species are separated,with M. paradoxus found further offshore than M. capensis. Complex retentionmechanisms enable larvae on the inshore drift route to reach the coastal nursery areaoff Cape Columbine, whereas larvae from the offshore route are transported furthernorth. This transport and dispersal mechanisms are important for larval connectivity.Therefore, the spawning strategies of the two hake species are adapted to the variable environment of the southern Benguela system, as spawning takes place during a time ofoptimal transport conditions for eggs and larvae towards their specific nursery areas.In addition, factors affecting the recruitment of hake were investigated by analysinggrowth and condition of early stages. Analyses indicated that the South African hakesare fast growing species. Survivors outgrow their conspecifics and cannibalism seemsto play an important role in the survival strategy of hake. Larvae were generally in goodcondition, as indicated by high RNA:DNA ratios, and they can be regarded as thesurvivors, as starving larvae with poor condition are more vulnerable to predation.RNA:DNA ratios as condition proxy and growth did not correlate in hake larvae, as theRNA:DNA ratios were well above the threshold level for growth. Furthermore, low lipidcontents were found in yolk-sac larvae of one investigated year, indicating maternaleffects by poor conditions of spawners, possibly affecting recruitment. The life historystrategy of hakes can be described as periodic with adaptations from an opportunisticstrategy.In conclusion, the interdisciplinary approach of this study provided new and importantknowledge on the early life history and the recruitment of the Southern African hakespecies, which can be transferred to other demersal, long-lived and fast-growing fishspecies. Furthermore, this information can be applied to fisheries management and itcan help to predict the consequences of climate change for M. paradoxus and M.capensis in the southern Benguela upwelling system.