Search

This study analyses the effect on sustainability of a strategy of exploitation under age-specific survival probabilities fixed through time. Exploitation can be considered an artificial selective mortality that affects the stability of ecological communities. For the exploitation of any single species, it is necessary to obtain some indices that permit to check when the exploitation strategy may become an overexploitation strategy. For fisheries, supposing an asymptotically stable equilibrium exists, the number of recruits per spawning biomass units at the steady state need be bounded, such a boundary being dependent on reproductive parameters. In a previous study, this is shown to be a necessary condition for sustainable exploitation. Also, it was shown that when the stable age structure is “correctly balanced” then the necessary condition for sustainability is verified. Experience say that in order to maintain the sustainability, it is necessary to have diversity of individuals in the age structure. In this paper, the analysis of how the stable age structure should be, relative to reproduction parameters in order to induce the population collapse is performed. The results show that if the age-specific survival probabilities for ages that contribute to recruitment are too low, then the population tends to extinction. Assuming a stable age structure, there exists an extreme value ω 1 ∗ , for the first component of the stable age structure (ω1), related to the age-specific survival probabilities that do not contribute significantly to recruitment. When these survival probabilities are low, ω 1 ∗ is large and conversely. If ω 1 >ω 1 ∗ then maintaining through time the exploitation strategy, or in other words the age-specific survival probabilities, forces the population to the extinction. This is a sufficient condition for a nonsustainable exploitation strategy. In addition, this result shows that an unbalanced stable age structure as characteristic of nonsustainability. Besides, it is possible to see that the necessary condition for sustainable exploitation is not a sufficient condition. We take the Argentine hake (Merluccius hubbsi) fishery as an example where the necessary condition is true and, nevertheless ω 1 >ω 1 ∗ . The analysis allows to reach the conclusion that during the period 1986–1999 this fishery stock was overexploited. The loss of adult specimes and depletion of spawning stock biomass, biomass, and numbers of individuals are characteristic of that time period. Also in this paper, a proposal for an exploitation strategy is analysed, through which recovery and sustainable exploitation for this hake stock are expected. This exploitation strategy contemplates a specific fishing gear and suitable closed areas.

The young-of-the-year (YOY) Argentina hake Merluccius hubbsi remained at particular distances off the seabed at pelagic habitat, in a nursery area located in the San Jorge Gulf off south-western Atlantic. Older specimens were daylight distributed near the bottom in the demersal habitat. In this paper the authors show that the pelagic habitat is favourable for YOY hake compared to the surrounding demersal habitat by decreasing the probability of encounter with larger hake protecting the YOY against conspecific predation, and providing a better food supply. From 303 stomachs of pelagic YOY Argentine hake analysed (60-250 mm), 274 (90.43%) contained prey. Pelagic Argentine YOY hake fed almost exclusively on zooplanktonic crustaceans (Euphausia spp. and Themisto gaudichaudii). From 980 demersal specimens (14-82 cm), 572 (58.36%) contained prey, mainly the lobster krill Munida gregaria, followed by other hakes by cannibalism. The intensity of feeding was higher on pelagic layers. The allometric weight-length relationship revealed that the YOY Argentine hake find sufficient food in the pelagic habitat to live and develop, by increasing their relative body thickness. The cannibalism increases from close to zero when the YOY hake are c. 20 m off the seabed, to between 15% (cold season) and 20% (warm season) when they are 10 m off the seabed. These results suggest that the pelagic habitat is a feeding ground for YOY hake, and it is a favourable one compared to the surrounding demersal habitat by protecting the YOY from cannibalism. Pelagic YOY hakes were less abundant and more distant from the bottom during the cold season (14.3 m) than during warm one (11.4 m), probably because of natural mortality and progressive recruitment to demersal habits.

The characterization of the acoustic recordings obtained by an echo sounder is the first step in the process of identification and classification of marine organisms. This technique is essential for abundance estimation or for detailed ecological studies using acoustic methods. This paper presents echo recordings of Argentine short-fin squid (Illex argentinus) concentrations, collected during February in three pre-recruit bottom trawl assessment surveys (2006, 2007, and 2009) carried out by the INIDEP (National Institute for Fisheries Research and Development). The survey area was a region on the Patagonian Shelf (100–200 m depth) between 44°S and 51°S where pre recruits of I. argentinus South Patagonian Stock (16–29 cm ML) were feeding on dense aggregations of zooplankton. Advanced techniques were used for post-processing the acoustic data, including the generation of synthetic echograms and the use of recognition algorithms of geometric patterns. Data were collected at two ultrasonic frequencies: 38 and 120 kHz. Typically, squid pre-recruits aggregated in schools located on or near the bottom. Squid concentrations varied in size, shape, and volume backscattering coefficient (average S v ). Squid schools echo traces usually overlapped with those of predators such as Argentine hake (Merluccius hubbsi) and prey such as amphipods (Themisto gaudichaudii). Results show how the frequency response of the sound scattering can help characterize the squid concentrations and differentiate them from the other main scatterers in the area. The average differential scattering of S v ( 120 – 38 kHz ) was +5 dB for squid, while values of +18 dB and −1.6 dB corresponded to the amphipods and hake, respectively. Statistical analysis supported the validity of the differentiation method.

The present paper deals with the spawning characteristics and the fecundity of the Argentine hake, Merluccius hubbsi from the zone in front of the Buenos Aires Province. More than 500 ovaries were studied and 168 of them were considerd in the statistical calculations.