ANÁLISIS DEL MERCADO

This study investigated the relationship between zooplankton biomass and larval fish abundances, with the former regarded as a proxy for secondary production in the Tamar Estuary. Peaks in zooplankton biomass occurred in November, both in 2001 and November 2002, when temperatures were ~15°C. This spring peak, some two months before the attainment of maximum temperatures (~19°C), parallels that in other temperate estuaries worldwide (Table 7.1). Larval fish concentrations also peaked in November both in 2001 and 2002, similar also to the situation with other temperate estuaries, where peak larval fish concentrations occurred 2-4 months before peaks in temperatures (Table 7.1).

Table 7.1. Estuaries (E) and enclosed bays (EB) where concentrations of zooplankton biomass (ZB) or larval fishes (LF) peaked during late-spring/early-summer, i.e. 2-4 months before the annual peak in water temperature.

System E/EB LF/ZB References

Long Island Sound (United States) E ZB (Capriulo et al., 2002) Chesapeake Bay (United States) E ZB (Roman et al., 2005)

Seine (France) E ZB (Mouny and Dauvin, 2002)

Gironde (France) E ZB (Sautour and Castel, 1995) Ems (Netherlands) E ZB (Sautour and Castel, 1995) Westerchelde (Netherlands) E ZB (Sautour and Castel, 1995) Swan Estuary (Australia) E LF (Neira et al., 1992) Botany Bay (Australia) EB LF (Steffe and Pease, 1988) Algoa Bay (South Africa) EB LF (Beckley, 1986) Whangateau Harbour (New Zealand) EB LF (Roper, 1986)

The match between peaks in zooplankton biomass and larval fish concentrations in the Tamar Estuary contrasts with the mismatch described for the Hopkins River in Australia and Newport River in United States, where zooplankton peaked 1-2 months before larval fish concentrations (Thayer et al., 1974; Newton, 1996). This suggests

Chapter 7: General discussion

that the timing in the larval fish occurrence in the Tamar is strongly related to both increasing temperature and zooplankton production cycles, both of which are key factors in triggering reproductive activities of temperate fishes (Bye, 1984; de Lafontaine et al., 1984).

No evident difference in the spatial distribution both of zooplankton biomass and larval fish concentrations was found in the Tamar Estuary, following examination of these data using the Venice system of salinity regions. In the case of zooplankton biomass, this lack of evident spatial pattern contrasts the findings in temperate estuaries such as the Seine, Gironde, Ems and Westerschelde, where the greatest biomasses were recorded in the mesohaline or oligohaline region (Sautour and Castel, 1995; Mouny and Dauvin, 2002). Likewise, the lack of a spatial pattern in larval fish concentrations between salinity regions contrasts that found in several South African estuaries, where the greatest concentrations were obtained in the mesohaline (5-17.9 PSU) region (Strydom et al., 2003). A number of factors could affect the distribution of zooplankton biomass and larval fish concentrations in the Tamar Estuary leading to a lack of spatial pattern, including the strong tidal currents (~2 m/s) which may be responsible for redistributing concentrations more uniformly along the estuary. While spatial differences in larval fish concentrations have also been reported in other temperate estuaries worldwide, none of these studies used the Venice system thus making it difficult to draw comparisons with this study.

The consistent timing in the annual peak in larval fish concentrations recorded in the Tamar Estuary implies a regularity in the spawning time. This in turn suggests that a number of fish species in the Tamar may have fixed spawning periods to ensure a

match between larvae and food supply (Cushing, 1975; Bye, 1984). Spawning regularity has also been reported for other temperate estuaries, where peaks in larval fish abundance occurred during the same month across different years (de Lafontaine et al., 1984; Drake and Arias, 1991; Witting et al., 1999).

Considering the occurrence of larvae, the main spawning season of some estuarine fish groups in the Tamar (e.g. gobiids, blenniids and anchovy), appeared to last ~2 months, as indicated by the fact that the peak in larval fish concentrations lasted only two months (November to December) before decreasing rapidly. This rather short spawning season was clearly predicted in the 1-dimensional transport model as an exponential decline with an decaying time of ~5 days (see Chapter 6). The close correspondence between the simulated and field-obtained larval fish concentrations suggests that the model representation of a short but intense spawning season was reasonable in the case of these fish groups. The comparatively short spawning season in the Tamar Estuary differed from that reported in other Australian temperate estuaries where larvae have been caught over a protracted period of time, i.e. 4 months, such as the Hopkins River, Swan Estuary, Nornalup-Walpole Estuary and Wilson Inlet (Gaughan et al., 1990; Neira et al., 1992; Neira and Potter, 1992b, 1994; Newton, 1996).

Another aspect of the Tamar Estuary that may be relevant to larval fishes is the presence of big swarms of large jellyfishes during March and April 2002, at the time when concentrations of larval fishes and zooplankton biomass were very low. Although it is not known whether these jellyfishes represent one or more species, significant predation by jellyfish on larval fishes and other zooplankters has been

Chapter 7: General discussion

reported in several temperate estuaries worldwide (Fancett and Jenkins, 1988; Newton, 1996; Esteves et al., 2000; Capriulo et al., 2002; Roman et al., 2005). The presence of these jellyfishes at the time of low larval fish abundances suggests that the spawning timing and duration of fishes in the Tamar Estuary may be influenced to some extent by predator presence. This observation is supported by the fact that both eggs and yolk-sac stage larvae are more vulnerable than postlarval stages as they are unable to avoid predatory jellyfishes (Fancett and Jenkins, 1988; Esteves et al., 2000). In addition, it has also been suggested that the historical impact of predation in some fish species may have modified the reproductive behaviour of adults in order to reduce larval loss due to predation (Frank et al., 1982; Esteves et al., 2000). In the case of the Tamar, however, it is still unknown whether larval survival depends on density-independent (e.g. temperature and advection) or density-dependent (e.g. starvation and predation) mechanisms, or both (Bailey and Houde, 1989; Houde, 1997; Esteves et al., 2000). In any case, it is possible that both the start of the spawning season as well as the intensity of spawning in the Tamar are related to abiotic factors, such as increasing temperature and moderate freshwater flow, whereas spawning duration may be linked to biotic factors, such as presence of potential predators and food availability.