Seasonal and short spatial patterns in European hake (Merluccius merluccius L.) recruitment process at the Balearic Islands (western Mediterranean): The role of environment on distribution and condition

This study evaluates the link between the recruitment process of European hake (Merluccius merluccius L.) of the Balearic Islands (western Mediterranean) and the environmental and physiological conditions. Spatio-temporal variation of abundance and condition of fish were evaluated at two locations each with different oceanographic conditions, one in the north (Sóller, SO) and another in the south (Cabrera, CA) of Mallorca Island. Environmental variables explored were hydrography, sediment characteristics, phytoplankton pigment concentration (ppc) and the trophic resources of hake. Individuals were divided in three life stages: recruits, post-recruits and young adults. Hepatosomatic index (HSI), relative condition index (K_n), gonadosomatic index (GSI) and fullness index (FI) were analysed for the three life stages. Recruitment starts in February with the incorporation of smaller hakes, and it can be followed through spring and early summer with a peak in April. However, some spatial heterogeneity in the recruitment process has been found between north and south of the Island. The main pulse of recruitment occurred at a different time in the two areas. Spatial heterogeneity was also consistent with the condition of hake recruits, with higher values of K_n and HSI at SO than at CA. Maximum values of K_n were found in February at SO and in April at CA, coinciding with the start of the different recruitment pulses to the fishing grounds. Post-recruits and young adults also showed higher condition at SO than at CA. The arrival in spring of the Western Winter Intermediate Waters (WIW) drives the spatial-temporal variation in abundance and condition of hake. Ppc was highly correlated with recruit abundance with a time lag of two months, while for post-recruits the time lag was three months. The observed differences in the condition of hake between areas could be a consequence of the fact that the waters to the north of Mallorca are comparatively more under the seasonal influence of WIW which is formed in more productive areas. Thus, this study characterises the short temporal and spatial variability in the hake recruitment process off the Balearic Islands, both in terms of abundance and fish condition. This pattern is explained on the basis of the mesoscale environmental variability observed between north and south of Mallorca and the ecological adaptive strategy of recruiting in the optimal environmental season.     

Population dynamics of the red shrimp Aristeus antennatus in the Balearic Islands (western Mediterranean): Short spatio-temporal differences and influence of environmental factors

The red shrimp Aristeus antennatus is one of the target species of the bottom trawl fishery of the Balearic Islands. The objective of the present paper is to study the short spatial and temporal differences of this important economic resource between two different locations off Mallorca (Cabrera: CA; Sóller: SO), where a fleet mobility pattern has been detected, and to study the influence of environmental conditions on this species. Six simultaneous bottom-trawl and oceanographic surveys were carried out at these two locations in order to collect data from the demersal species, hydrography (temperature and salinity), trophic resources and sediment characteristics. The commercial fleet from both locations was monitored by monthly on-board sampling, log-books and daily landings obtained from sales slips. Additional data was obtained from other fishing surveys. Short spatial and temporal differences have been detected between both locations. The population at CA was more demographically homogeneous, while that at SO showed important variations, like high abundance of juveniles recruiting to fishing grounds in autumn–winter and high abundance of large females during summer. Several differences have also been found in the biology of the species between locations, such as males were more abundant in SO than in CA. Also, the reproductive period started sooner in SO than in CA, and the condition of pre-spawning females was better in SO. The percentage of total lipids in the hepatopancreas was minimal during the spawning period, showing their importance as a reserve of energy for the ovary ripening. Water masses could play an important role in these differences, the characteristics of water masses being more stable in CA than in SO. Red shrimp adult females seemed to be more correlated with the warmer and more saline Levantine Intermediate Waters, while juveniles (males and females) and adult males were more correlated with the colder Western Mediterranean Deep Waters, detected only in SO during autumn–winter. Two different hypothesis of mobility patterns for the species are discussed in relation to these observed differences.     

Spatio-temporal variations in deep-sea demersal communities off the Balearic Islands (western Mediterranean)

The spatial and temporal variations of deep-sea megafaunal assemblages from the western Mediterranean are analysed in the present paper. The assemblages from two locations of the Balearic Islands situated 120 km apart were compared using data collected seasonally on a bathymetric stratum covering the 150–750 m depth range during six bottom-trawl surveys. The assemblage structure, in terms of species composition, species dominance and population sizes, was differentially affected by the spatio-temporal variables analysed (depth, location and fishing period). Although depth was the main factor determining the assemblage composition, the differences obtained between the two locations were also relevant. On the upper slope these between-location differences in the dynamics of megafaunal assemblages were found to be related to the effect of fishing exploitation. Population size-based metrics and biomass spectra were good predictors of meso-scale fishing effects, and were mainly reflected by elasmobranchs and demersal teleosts. Nevertheless, the effects of fishing depended on the species considered. Two dominant large-sized fish species found on the upper slope in both localities, Galeus melastomus and Phycis blennoides, had higher biomass values associated with lower fishing effort. Although the mean body weight (MBW) of both species and also the mean maximum body weight (MMBW) of G. melastomus agreed with this pattern, the P. blennoides MMBW did not. This last case could be indicative of natural size-trends such as the bigger–deeper phenomenon which refers to the displacement of large individuals towards the deeper limit of their bathymetric distribution, beyond the maximum depth sampled in this study for this species. By contrast, the target species of the upper slope fishery, the red shrimp Aristeus antennatus, was not negatively affected by the direct impact of fishing activity and other environmental factors, such as the presence of specific water masses could also be important.     

The influence of oceanographic scenarios on the population dynamics of demersal resources in the western Mediterranean: Hypothesis for hake and red shrimp off Balearic Islands

The aim of the present paper is to study the relationships between some climatic indices and parental stock, recruitment and accessibility to trawl fishery of hake (Merluccius merluccius) and red shrimp (Aristeus antennatus) off Balearic Islands (western Mediterranean). Available annual catch per unit effort, recruitment and spawning stock biomass have been used as biological data. As environmental data, the meso-scale IDEA index and the large-scale North Atlantic Oscillation (NAO) and Mediterranean Oscillation (MO) indices have been used. To analyze possible links between these indices with the population dynamics of demersal resources, two non-linear approaches have been applied: (i) stock–recruitment relationships from Ricker and Beverton–Holt models, by sequentially incorporating environment factors; (ii) generalized additive modelling, both classical general and threshold non-additive models were considered. The latter simulate an abrupt change in explicative variables across different phases (time periods or climatic index values). The results have shown that two oceanographic scenarios around the Balearic Islands, associated with macro and meso-scale climate regimes, can influence the population dynamics of hake and red shrimp. This is especially true for recruitment, which seems to be enhanced during low NAO and IDEA indices periods. During these periods, colder-than-normal winters generate high amounts of cold Western Mediterranean Intermediate Waters (WIW) in the Gulf of Lions, which flow southwards and reach the Balearic Islands channels in spring, increasing the productivity in the area. This oceanographic scenario could also be favourable to the distribution of hake on the fishing grounds where the trawl fleet targets this species, increasing its accessibility to the fishery. Both spawning stock and abundance of red shrimp seems to be also enhanced by high MO index periods, which could reflect the increased presence of the saline and warm Levantine Intermediate Waters (LIW) in the study area, extending over the fishing grounds of this species. The proposed interactions can be useful to assess and manage these important demersal resources.