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Ocean acidification and increased ocean heat content has direct and indirect effects on marine organisms such as holothurians (sea cucumbers) that are vulnerable to changes in pH and temperature. These environmental factors have the potential to influence organismal performance and fitness at different life stages. Tropical and temperate holothurians are more vulnerable to temperature and pH than those from colder water environments. The high level of environmental variation observed in the oceans could influence organismal responses and even produce a wide spectrum of compensatory physiological mechanisms. It is possible that in these areas, larval survival will decline by up to 50% in response to a reduction of 0.5 pH units. Such reduction in pH may trigger low intrinsic growth rates and affect the sustainability of the resource. Here we describe the individual and combined effects that temperature and pH could produce in these organisms. We also describe how these effects can scale from individuals to the population level by using age-structured spatial models in which depensation can be integrated. The approach shows how physiology can improve the conservation of the resource based on the restriction of growth model parameters and by including a density threshold, below which the fitness of the population, specifically intrinsic growth rate, decreases.

The improvement of survival in some species due to the close coexistence between individuals is a concept that has caught the attention of scientists over the years. This behaviour has been documented in many taxa, including species which support commercial fisheries. In recent years, a better understanding of the mechanisms associated with the Allee effect has shown its importance in population dynamics and probably plays a more important role than previously thought. In this article, the Allee effect was analysed by means of a spatial bioeconomic model. The Allee effect component was the positive relationship between the mating probability and the density of spawners. A depensation function with an offset to the right side of the origin represented the fraction of females that mated based on density. When the model did not include the Allee effect, the fishery reached the bioeconomic equilibrium; however, when the Allee effect was considered, the fishery did not reach equilibrium, but the stock collapsed in 15 years. Fishing mortality altered the spatial distribution of the stock, causing depensation in recruits per spawner. These results may contribute to a better understanding of the Allee effect on sedentary resources and can be used to explore new models.

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