The influence of habitat structure on energy allocation tactics in an estuarine batch spawner
Introduction
Transitional water systems, such as estuaries and coastal lagoons, typically show high degree of heterogeneity, due to the presence of multiple habitat types and the high spatio-temporal variability of environmental conditions (Irlandi and Crawford, 1997). This remarkable level of heterogeneity does not only support a large fish production (Elliott and Hemingway, 2002), but it is also expected to shape the life history strategies of fish populations by influencing many biological interactions, such as foraging behaviours, competition and predation (Irlandi and Crawford, 1997). According to Reznick et al. (2002), the optimal life history strategy is largely shaped by environmental factors that are represented by both extrinsic mortality schedules, related for example to predation, and by intrinsic and energetic habitat constraints. A number of empirical studies suggest especially the primacy of predation pressure in shaping life history strategies in small-sized freshwater fish species, occurring in fragmented populations subjected to different habitat characteristics (Jennions and Telford, 2002, Johnson and Belk, 2001, Reznick and Endler, 1982, Reznick et al., 2001, Reznick et al., 2002, Rodd and Reznick, 1991, Walsh and Reznick, 2008, Walsh and Reznick, 2009, Walsh and Reznick, 2010a, Walsh and Reznick, 2010b). Similarly, estuarine resident fish, whose isolated populations occur into different habitat types, represent an optimal model to test for the effects of contrasting habitat characteristics on the optimal life history strategy. Within the Mediterranean and Northern American salt-marsh habitats, killifish constitute an excellent candidate for these kind of studies, as they are small-sized, resident fish, occurring in fragmented, shallow water habitats characterised by variable degree of structural complexity, confinement and food richness. Recently, the relationships between habitat structure and life history tactics have been investigated in the Mediterranean killifish Aphanius fasciatus (Valenciennes, 1821) (Cavraro et al., 2014a), suggesting that the optimal life history strategy is mainly shaped by food richness and morphological structure of the habitat. In particular, this study indicated that a different degree of confinement, by influencing the presence of predator and thus adult mortality, would influence the patterns of energy allocation. The two sites selected to compare the energy allocation tactics were characterised by high and comparable level of richness in basal resources (similar level of organic matter in the sediment), but showed two different mortality schedules: an open natural salt marsh (Campalto, CA) exposed to high level of predation, due to the connection with the open lagoon, and a confined, artificial site (Vignole, VI), composed of a system of small ditches, protected from piscivorous predation (Cavraro et al., 2014a). Despite having similar fish densities, these sites differed strongly in mortality, showing an overall mortality of 57% in CA and 28% in VI (Cavraro et al., 2014a), confirming that the degree of confinement is able to influence the susceptibility of this species to predation. Results showed that fish experiencing higher predation pressure, and thus suffering from higher adult mortality, presented a higher reproductive allotment and a shorter life span (Cavraro et al., 2014a). On the contrary, predation intensity seemed not to influence the age/size at maturity, although this species shows a precocious maturity and differences in the magnitude of days or months were not easy to be detected with scale reading. Furthermore, a major peak in the gonado-somatic index was observed during the spawning season in the site subjected to tidal influence (open, natural salt marsh) while the artificial creek showed a lower reproductive allotment, that was apparently allocated into more different GSI peaks within the breeding season (Cavraro et al., 2014a). According to traditional life history theories (Stearns, 1992, Roff, 1992), higher investment in reproduction should have a negative influence on survival. The energy allocation tactic in the open natural saltmarsh should therefore favour a higher reproductive investment at the expense of survival, decreasing the fitness of older fish, accelerating growth, and reducing the number of spawning events over the life span. By contrast, in the confined habitat, where reproductive allotment is higher, slower growth rate, higher number of spawning events and higher fitness of older individuals should be favoured.
To test for these habitat-dependent differences in energy allocation, a bioenergetic model is here applied, using the previously collected empirical data (Cavraro et al., 2014a). The model, developed and calibrated for the specific goals of this paper, could be a proper tool to assess and compare the energy allocation tactics characterising the average individual life history in the two contrasting habitats. While the previous empirical work showed evidence of differences in reproductive allotment within a single breeding season, the present bioenergetic model should allow calculation of the number of reproductive events, the per-event reproductive investment and the somatic growth rate over the entire life span of the species in the two contrasting habitats.
Section snippets
Materials and methods
A standard bioenergetic model formulation (Ursin, 1967) was modified to capture A. fasciatus batch spawning dynamics. In accordance with the modelling strategy adopted by Wang and van Cappellen (1996), model parameters were divided in two categories:
- i)
reaction specific parameters, such as metabolic rates and temperatures, considered independent from the site, and set on the basis of previous studies on the eco-physiology of this species;
- ii)
parameters governing reproduction, which were considered as
Comparing the average individual life histories between the two sites
The results of the calibration of the k parameter are presented in detail in Appendix B. Fig. 2 and Table 3 show a comparison among two single model runs, which were performed with average values of calibrated k (i.e. 0.42 in CA and 0.44 in VI). Two different dynamics of G can be observed:
- i)
in CA characterized by at maximum 3 important events per year (years IV and V have only 2 events), and spawning concentrated in the last part of the reproductive time window (marked in grey in the figures);
- ii)
in
Life history differences between the two contrasting habitats
The average individual life history strategy of Aphanius fasciatus, as resulting from the application of a SfG model to the empirical data obtained by Cavraro et al. (2014a) in the two contrasting habitats of CA and VI, revealed that fish in VI invest slightly more energy in reproduction than in somatic growth over the entire life span. In this site, fish allocate the reproductive energy into more spawning events (20 vs 12), being the investment for single spawning event smaller in comparison
Conclusions
The application of the SfG model strongly suggested the central role of energy partitioning through batch spawning in determining the life history strategy adopted by female A. fasciatus. The particular ovary structure of this batch spawner seemed therefore to allow fish to modulate the timing and the investment of spawning events, shaping the optimal life history in relation to the environmental conditions.
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