Ana Gordoa

Age and growth of the sparids Diplodus vulgaris, D. sargus and D. annularis in adult populations and the differences in their juvenile growth patterns in the north-western Mediterranean Sea

Abstract Age and growth patterns of adults of three common species of sparids, Diplodus vulgaris, D. sargus and D. annularis from the north-western Mediterranean Sea are analyzed. Length-at-age values are estimated from otoliths and growth patterns are well described by the von Bertalanffy growth equation. The sampled length interval ranged from 8–40, 28 and 20 cm for D. sargus, D. vulgaris and D. annularis, respectively. The estimates of t0 for D. vulgaris and D. sargus, −0.67 and −0.74 respectively, indicate that the growth of these species during the first months of life do not adhere to the von Bertalanffy model. In contrast, for D. annularis the estimation of the origin of the von Bertalanffy model is close to zero (t0 = −0.03), suggesting that this species is faithful to this growth model from the beginning of settlement. The variations from juveniles to adults in the growth pattern are explained by the permanence or the variations of the habitat of each of the species throughout their life.

Growth performance of four wrasse species on the north-western Mediterranean coast

Age and growth patterns of four common species of wrasses, Labrus merula, Coris julis, Symphodus roissali and Symphodus tinca, from the north-western Mediterranean sea are analysed. Length-at-age values are estimated from otoliths from which the first annual hyaline ring is determined by reading daily rings. The growth patterns are well described by the von Bertalanffy growth equation with the exception of S. tinca which showed an extremely high dispersion in the length-at-age distribution. The daily rings show the second hyaline ring as the first annual hyaline ring, whereas the first hyaline ring was formed after 2 months of life coinciding with the settlement period of these species, which indicates that changes in otolith crystal growth during the first year of life could be driven by biological or physiological changes in the individuals.

Influence of Benguela upwelling on the structure of demersal fish populations off Namibia

The effect of bottom-water dissolved oxygen on the structure of demersal fish communities off south Namibia was analized. Collections were made during two cruises of the zone in winter 1987 and summer 1988, i.e., before and after the intensive upwelling season. Multivariate analysis revealed four distinct associations, separated by latitude (ca 27°S) and depth (ca 300 m isobath), indicating that dissolved oxygen is an important factor affecting the distribution of demersal fish communities in the zone. Faunal diversity decreased in areas of low oxygen levels. The seasonal variation in upwelling reflected the extent of fish association, their composition, and the seasonal geographical displacement of their boundaries.

Facts and uncertainties about the genetic population structure of Atlantic bluefin tuna (Thunnus thynnus) in the Mediterranean. Implications for fishery management

The Atlantic bluefin tuna (Thunnus thynnus) is an extraordinary fish that has amazed humanity since ancient times. However, the continuous overexploitation of this fishery, particularly in the Mediterranean Sea, could result in a total collapse of this resource. Currently, this species is managed as two stocks: Western Atlantic and Mediterranean-Eastern Atlantic, with a recognized genetic differentiation between them. On the other hand, the population structure within the Mediterranean Sea is still unclear. The biological data supports the idea of two separate populations in the eastern and western Mediterranean basins. However, nuclear microsatellite and mitochondrial DNA (mtDNA) analyses of two samples representative of these two basins result in a lack of heterogeneity. A comparison of these results with previously published studies reveals some discrepancies. We have compared 59 genetic differentiation tests that include samples within the Mediterranean. Of these, about 60% gave significant differentiation while the remaining 40% were non-significant. But, when only nuclear-based loci were considered, genetic differentiation was detected in up to 73% of the cases with an average significant FST of only 0.018, whereas the average significant FST of the mtDNA-based studies was significantly higher (0.029). However, in some cases, it is difficult to reconcile the biology of the species with the results suggesting genetic differentiation. In conclusion, although it is not yet possible to reach a definitive conclusion about the population structure, but considering all biological and genetic data, we suggest an independent management approach for each basin to avoid the impact of a type II error that could lead to the possible loss of the regional subpopulations.

Diurnal variation in the feeding activity and catch rate of cape hake (Merluccius capensis and M. paradoxus) off Namibia

Abstract The diurnal pattern in the catch rate and feeding activity of Namibian hakes ( Merluccius capensis and Merluccius paradoxus ) was studied. Merluccius paradoxus fed more actively during the night, whereas the opposite was the case for M. capensis . However, catch rates for both species followed the same diurnal pattern. At night, the availability of these species to bottom trawls was lower, although the difference was relatively small (night cathces were 85–90% of day cathces). The diurnal pattern of the length composition of the catches for both species was constant and these patterns were maintained throughout the year.

Trends in the demersal fish community off Namibia from 1983 to 1990

Demersal fish communities off southern Namibia were analysed on the basis of seven winter and five summer stratified random sampling cruises between 1983 and 1990. The 37 most abundant species selected for analysis represented 78-n the fluctuations recorded on the summer cruises appeared to be more closely related to hydrographic conditions. In relatively cool summers the estimated biomasses of seven species (including Merluccius capensis and Lophius vomerinus) were quite similar to, or lower than, the biomasses estimated six months earlier. In contrast, the estimated biomasses of the same seven species during relatively warm summers were distinctly higher than the estimates made the previous winter. In all, 10 species, including the commercially important Austroglossus microlepis, Lophius vomerinus and Merluccius capensis, were mo...

Monthly variability in the catchability of Namibian hake and its relationship with environmental seasonality

Abstract The monthly and spatial patterns of the Namibian hake fishery are analysed for the period 1994–1997. A strong, but misleading negative relationship between monthly catch rates (CPUE) and fishing effort was observed, while no spatial correlation between these two variables were found, which suggests, for this fishery, that monthly CPUE as an index of density is not distorted by fleet distribution. A strong seasonal pattern of catchability was found for the first three years but not in 1997. Catch rates could change by a factor of up to three in a period of six months. Moreover, the seasonal pattern in catchability occurs over the whole area, which excludes migratory displacements as a possible factor of this seasonality. We found a strong correlation between CPUE and sea surface temperature (SST). Thus we can conclude that hake catchability followed the seasonality of the system but failed when the seasonal cycle was weakest (i.e., 1997). The interannual variability of either the system or catchability is much larger during the summer than the winter, which has direct implications for resource monitoring and management.

Effect of prey densities on cannibalism in Cape hake (Merluccius capensis) off Namibia

The preference of large (>60 cm total length) Cape hake (Merluccius capensis Castelnau, 1861) for small conspecifics under natural conditions was analyzed off southern Namibia (S.E. Atlantic) between 1983 and 1989. The results indicate that cannibalism is not related to the density of the small conspecifics nor to the density of alternative prey, but rather that large hake have a dietary preference for small conspecifics. The main consequence of such cannibalism is a lack of density-dependent regulation of the M. capensis population.

Biology and fisheries of Namibian hakes (M. paradoxus and M. capensis)

The Benguela current system is located off the South-east Atlantic coast of Africa between 17°S and 35°S. This is one of the four major eastern boundary current regions in the world’s oceans. Coastal upwelling is the most important feature in such systems, in which high primary productivity supports large commercial fisheries (Cushing, 1971). Many aspects of the Benguela ecosystem have already been reviewed: description of the physical features and processes of the Benguela ecosystem (Shannon, 1985); chemistry and related processes (Chapman and Shannon, 1985); plankton (Shannon and Pillar, 1986); the major fish and invertebrate resources (Crawford et al., 1987); and the coastal zone (Branch and Griffiths, 1988).

Estimating the probability of different levels of recruitment for Cape hakes Merluccius capensis off Namibia, using environmental indices

Abstract This paper describes the use of multinomial logistic regression analysis to calculate the probability of strong, average or weak recruitment of Cape hake ( Merluccius capensis ). The model includes environmental indices, describing the extent of warm-water intrusion as well as the upwelling strength for different areas and periods over 2 years—from spawning until the fish are approximately 2 years of age. One of the warm-water indices as well as two of the upwelling indices have significant influences on the recruitment strength and these indices are included in the model. The rationale for using these indices is that if warm water is widespread during the spawning period (September–March) and the upwelling during the following May–September is reduced, the eggs spawned would be retained in favorable nursery areas which may result in potentially strong recruitment. If the upwelling during May–September of the following year produces sufficient food for the late juvenile hake, the chances of strong recruitment will be increased. The results indicate that the model accounts for 79% of the variance in recruitment.