Browsing by Author "Defeo, O"

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    Effects of harvesting and density dependence on the demography of sandy beach populations
    (1999) Brazeiro, A; Defeo, O
    Sandy beach ecologists concur that natural populations living in these harsh environments are controlled almost exclusively by physical factors, biotic factors being largely irrelevant. This paper provides evidence that human-induced perturbations as well as biotic, density-dependent processes also influence sandy beach populations. Results were obtained from a long-term study (8 yr) of the artisanally harvested yellow clam Mesodesma mactroides on a Uruguayan exposed sandy beach. The study included an experimental manipulation of fishing effort based on the closure of the clam fishery for 32 consecutive months. Fine-scale demographic parameters, such as age-specific survival probability and fertility, as well as coarse-scale demographic parameters, such as age composition, elasticity to demographic parameters, adult clam density, and population growth rate, were significantly affected by fishing effort and/or adult density. These results yield useful information from the management and conservation points of view, such as the threshold range of parent stock densities and fishing mortality levels capable of supporting a sustainable yield over time. Critical demographic parameters for population growth rate are also highlighted.
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    Harvesting and economic patterns in the artisanal Octopus mimus (Cephalopoda) fishery in a northern Chile cove
    (1998) Defeo, O; Castilla, JC
    We analyze intra- and inter-annual harvesting and economic patterns of the cephalopod Octopus mimus (pulpo) artisanal fishery at Caleta Coloso cove, Antofagasta, northern Chile, between 1991 and 1996. Information was collected daily, from 25 hookah divers that manually harvest pulpo in the subtidal along 29 km of coastline. Inter-annual fluctuations in catch, fishing effort, unit price and economic revenues increased from 1993 to 1996, when O. mimus landing and total revenues at Coloso were over 77 t and US$ 190 000, respectively. Contrary to the observed annual trend, the inverse intra-annual correlation between catch/fishing effort and price indicated short-term changes in price according to resource availability. A monotonic decreasing function between CPUE and price suggested a threshold catch rate economically feasible for harvesting. Interannual changes in the slope of the price-CPUE function indicated a marked increase in the demand of pulpo, jointly with warnings of stock depletion. We conclude that the 0. mimus temporal fishery patterns at Coloso are scale-dependent. Our results and general fishery patterns in the Chilean pulpo fishery suggest that a precautionary management scheme is urgently needed to prevent overfishing. (C) 1998 Elsevier Science B.V. All rights reserved.
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    Is sandy beach macrofauna only physically controlled? Role of substrate and competition in isopods
    (1997) Defeo, O; Brazeiro, A; deAlava, A; Riestra, G
    Exposed sandy beaches have been defined as physically stressful environments, so that benthic populations living there are thought to be regulated mainly by physical factors, biological interactions being minimal. However, recent long-term studies indicate that potential intra-and interspecific interactions should also play a role in structuring populations and communities. This paper evaluates the role of sediment characteristics and potential interactions in determining the abundance and distribution patterns of the cirolanid isopods Excirolana armata and Excirolana braziliensis in sandy beaches of Uruguay. Results from concurrent field sampling and laboratory experiments showed that: (1) at a macroscale (between beaches), E. armata occurred only in beaches with fine sands, whereas E. braziliensis was observed in both fine and coarse sand beaches, reaching its highest density in the latter; (2) at a mesoscale (within beaches) and in sympatry (fine sands), both cirolanids showed maximum densities at different tidal heights, with E. braziliensis restricted to the upper beach levels; (3) both isopods showed a clear preference for fine sands, when tested in isolation or combined; (4) survivorship of E. armata was higher when tested in the preferred sediment under co-occurrence with E. braziliensis, which in turn presented higher survivorship in coarse sand, either in isolation or combined with E. armata; and (5) individual mean length of both species was consistently higher in allopatry, and similar body lengths were observed in sympatric populations. A geographical analysis of the abundance of E. brazilienis along Pan-American beaches showed that this isopod is most abundant in fine sands; this overall pattern supports conclusions derived from sediment preference experiments, implicating a greater niche breadth than that observed in Uruguayan beaches. It was concluded that E. armata could be defined as a high substrate-specific species in which intraspecific interactions would be of utmost importance in population regulation. However, distribution patterns off. braziliensis could not be explained by a simple animal-sediment relationship, and correlational evidence suggests that it is displaced by E. armata towards coarse sands and upper beach levels. Thus, potential biotic interactions cannot be discarded as a structuring force in sandy beach communities. (C) 1997 Academic Press Limited.
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    Population dynamics of the yellow clam Mesodesma mactroides: recruitment variability, density-dependence and stochastic processes
    (2000) Lima, M; Brazeiro, A; Defeo, O
    Recruitment of benthic marine invertebrates varies greatly at different spatio-temporal scales. For instance, population dynamics of the yellow clam Mesodesma mactroides at Uruguay exhibit large temporal fluctuations, mainly associated with recruitment. We used data from an 8 yr long-term study to develop an age-structured model to show that density-dependent and density-independent forces acting together can jointly explain the population fluctuations in a sandy-beach bivalve population of the yellow clam Mesodesma mactroides. The pure density-dependent deterministic model parameterised with empirical values estimated during the 8 yr study predicted stable dynamics. The dynamics of the deterministic skeleton was markedly influenced by the addition of a relatively small amount of stochastic variability to fertility rates. The yellow clam population dynamics seem to be driven by the combined forces of density-dependent and density-independent factors operating together. A combination of (uncorrelated) stochasticity in reproductive rates and asymmetric intercohort interactions (density-dependent recruitment and density-dependent survival rates) seems to be the key process generating large variability in recruitment.