An update on the reproductive biology of the Merluccius capensis (Shallow water hake) in the south coast of South Africa

Abstract

The reproductive biology of the Merluccius capensis was investigated in the south coast of South Africa for the period between 2014- 2016 and 2019. In the present study the whole-mount (WM) technique is introduced for field assessment, applying the digital auto-diametric method for maturity staging of female Cape hake (M. capensis). The updated maturity-scale-designed oocyte diameter (OD) thresholds for six maturity stages were given as ≤ 250 μm for the immature, 251 - 350 μm for the developing, > 351 μm for the spawning (which includes spawning capable, actively spawning and the regressing stages) and 100 - 300 μm for the regenerating stages. There was 100% correspondence between WM and histological assessments for the developing and actively spawning stages, with an overall agreement across all other stages reaching >85%. The mean OD were significantly different (P < 0.001) for all stages when combined stages were tested, however separate testing of testing of stages showed no significant difference between the developing and the regenerating stages (P > 0.05). Furthermore, the test of the mean OD of the leading cohort (the most advanced oocytes) from all maturity stages indicated that the actively spawning stage was clearly the largest, (P < 0.05). Features including the existence of de novo vitellogenesis, i.e., the continuous recruitment of previtellogenic oocytes to the developing pool of oocytes, reconfirmed M. capensis as an indeterminate spawner. Furthermore, the presence of many oocyte cohorts at one time, as observed in the present study, supports the observation that this species is also a batch spawner. Original outputs from the random forest model pointed at all tested features as best in testing compatibility between histology and the WM technique, though oocyte diameter, area, and axis length yielded the same shape in oocyte categories. Thus, this simple method can not only be effectively applied in the laboratory but also in the field for more basic research to improve maturity staging quality and the understanding of reproductive patterns. The fecundity of M. capensis for the three - year period (2014-2016) was also investigated through a dedicated quantitative analysis on the various stages of oocyte developmental phases which are inclusive of the previtellogenic (PVO), cortical alveoli (CAO), vitellogenic (VTO) and hydrated oocytes (HO). The established algorithms– rooted in principal physics–made it possible to successively estimate the single oocyte volume, the total occupied volume by phase and thereby the fecundity of these different oocytes present at a given point in the reproductive cycle. The HO were significantly highest in volume, while the PVO were significantly highest in abundance (Fec,i). The ratio Fec,i for CAO and VTO were the same or partly differ during the season. A minimum VTO threshold where the relative fecundity (RF) and the subsample volume intersect is determined as a point from which batch fecundity (BF) can be calculated using the volumetric formulae. Our calculation exercise suggested a mean RF of 283 (SE = 30) per gram ovary-free (somatic) body weight and a (BF) of 566 (SE = 98) thousands during the peak season of austral autumn. More generally, over the three years of study the grand mean RF was 259 ± 20 per gram somatic body weight, while the grand mean BF was 522 ± 64 thousands. At least at the population level M. capensis spawns all year round, with the two highest peaks seen respectively in austral autumn (March and May) and austral spring (August and September). Naturally, the gonadosomatic index (GSI) increased before spawning activity and decreased during the high peak of spawning. The biggest gonads, found in the biggest females, produced the greatest BF values, which indicates that this parameter (BF) is both gonad-size and body-size dependent. Essentially, this study advocates for vigorous investigations on earlier oocyte developmental stages as opposed to the current status of focusing primarily on the advanced stages in fecundity estimations. This study also presents the exploration of the life history parameters of female Merluccius capensis off South Africa (N = 1817) which are inclusive of the gonadosomatic index (GSI), length-at-maturity, length-weight relationships and condition indices (relative condition (k) and Fulton’s condition factor (K)). GSI was slightly higher in spring and autumn (≥7%) but could not be confirmed during winter- and summertime due to less access to data in those periods. The opposite was true for the actively spawning stage as GSI values were higher than 7 % when compared to other maturity stages. The length (L) at 50% maturity was around 38 cm (L50), though differences occurred between the two applied staging methods, histology and visual (macroscopic) classification, when L approached infinity. The latter method presented underestimated length at maturity values at the 75 and 95 percentiles (48 and 60 cm) compared to the corresponding percentiles given from histology (50 and 65 cm). There were trivial across-method differences in L50. However, we found a clear reduction in L50 in view of published information in prior years when this estimate was 48 (1985), 42 (2008), 53 (2011) and 24.8 (2015) cm. Overall, L explained 90% of the variation in whole body weight (W). As the bootstrapped, grand mean growth coefficient was b = 2.98, indicating a slight allometric growth function, there were no significant variations between years, though an isometric growth existed for 2016 with b = 3.0, whereas for 2014 and 2015 this b was 2.98 and 2.93, respectively. In terms of demography, females < 60 cm generally showed isometric growth (b = 3) with less eggs as opposed to allometric growth (b = 2.95) at > 60 cm which laid more eggs. The relative condition index (k = 1) exhibited higher values than Fulton’s K which was 0.80. Taken together, the maternal stock of M. capensis along the south coast seems to be in good condition, likely spawn throughout the year but we found that the macroscopic data tend to give biased maturity ogives. Spatial and temporal distribution revealed that the body size increased with water depth and was significant at p<0.05, though this relationship between the body size (cm) and water depth (m) was rather weak (r2 = 0.22). The vertical adjustment of the position in the water column between the immature to regressing stage appeared trivial, however, those in the regenerating stage stayed significantly deeper (P<0.05), specifically at about 225 m mean depth compared to 160-200 m for the other stages. M. capensis, as a serial spawner, tended to move upwards by about 25 m to release egg batches. Spawning females occupied the latter mentioned mean depths in April to May and in September, evidenced by the improved incidence of actively spawning females. A latitudinal shift was also apparent as the females moved northwards before spawning (35.4-35.3ºS), followed by spawning (34.9ºS), and subsequently returning southwards (35.2-35.4ºS). These latitudinal and longitudinal movements came together with relocations to the east at pre-spawning (21.7-21.8ºE), then even further east when spawning (22.8ºE) and then going back to the west (21.2ºE) after spawning. Thus, the various phases of the reproductive cycle of this species are characterized by highly specific spatiotemporal dynamics.