Strategies of Pomatoschistus minutus and Pomatoschistus microps to cope with environmental instability

Abstract

The populations of Pomatoschistus minutus and Pomatoschistus microps were studied from June 2003 to May 2006 in the Mondego estuary, in order to understand the ability of these species to withstand environmental variability. During this period a severe drought occurred (2005), with consequent lower freshwater runoff and higher salinity incursion into the estuary. Occasional abnormal high water temperatures were observed in 2003 and 2005. The fish populations were sampled monthly along an estuarine gradient, from which population density, structure, growth and biomass production was assessed. Pomatoschistus minutus distributed mainly at the most saline downstream area, while P. microps distributed within the whole estuary, in accordance to the salinity and temperature tolerances for the species. A clear decrease in density and production was observed for P. minutus in the dry year, with non-expressive recruitments and the quick disappearance of the parental cohort. For P. microps, the decrease was not so pronounced. No direct effects were attributable to the salinity and temperatures variations (resulting from the drought and high temperatures). Yet, higher predation pressure on P. minutus hypothesis was raised, as the salinity incursion increased the piscivorous marine adventitious species in the downstream areas. Pomatoschistus microps benefited from a wider temperature and salinity range tolerance, allowing the species to occupy different areas in the estuary and by this seemed better able to cope with the environmental conditionings during the 3-year studied period.

Estuarine production of resident and nursery fish species: Conditioning by drought events?

“…For Pomatoschistus minutus and Pomatoschistus microps, as the class interval for the length frequency analyses was lower than 0.25 cm, ANAMOD software package was used, which provided the modes and their standard deviation, and checked the reliability of the estimated parameters. This analysis is described in Dolbeth et al (2007a).…”

“…More information on the environmental and hydrological parameters (e.g. dissolved oxygen, pH, water transparency, chlorophyll a) is reported in Dolbeth et al (2007a) and Marques et al (2007).…”

“…Estuarine systems are extremely valuable for many marine fish species of the continental shelf, by providing important nursery grounds Martinho et al, 2007a;Nicolas et al, 2007), as well as for several other fishes, such as the resident ones, spending their complete life cycle within the estuary (Elliott and Dewailly, 1995;Dolbeth et al, 2007a). Frequently, these two ecological fish guilds (marine juvenile and resident) represent the major part of the fish community found in estuarine systems (Elliott and Dewailly, 1995;Martinho et al, 2007b).…”

“…Several of the marine juvenile fishes that occur in estuaries are commercially important, hence estuaries may be considered as essential ecosystems for the renewal of fisheries resources, acquiring also a direct relevant economic value for mankind (Houde and Rutherford, 1993;Cowley and Whitfield, 2002;Able, 2005;Nicolas et al, 2007). Regarding resident species, their role as intermediates in the estuarine food web turns them crucial for the overall dynamics and functioning of the estuarine system (Leitão et al, 2006;Dolbeth et al, 2007a). Yet, estuarine areas are highly affected by human activities such as habitat reclamation, water quality impoverishment and fishing activities Vasconcelos et al, 2007) and are known for their high natural environmental variability (Maes et al, 2004;Elliott and Quintino, 2007).…”

“…More details and comparisons of both Pomatoschistus spp. productions can be found in Dolbeth et al (2007a).…”