Coexistence of Anadromous and Resident Life History Styles of Pond Smelt, <Emphasis Type="Italic">Hypomesus nipponensis</Emphasis>, in Lake Ogawara,Japan, as determined by Analyses of Otolith Structure and Strontium:Calcium Ratios

Anadromous and resident forms of pond smelt, Hypomesus nipponensis, were found to occur in sympatry in Lake Ogawara, Japan. Profiles of Sr: Ca ratios from individuals could be grouped to two patterns (1) a resident pattern with low Sr: Ca ratios from core to edge and (2) an anadromous pattern with relatively low Sr: Ca ratios near the core with abrupt increases in ratios at a location approximately 0.3mm from the core. Spawners smaller than 60mm standard length (SL) were resident, between 60 to 80mm were mixed resident and anadromous, and larger than 80mm were anadromous. Anadromous individuals first migrated after 40 to 82 days from hatching (mean±sd, 59.1±13.5d) and 14.6 to 30.9mm SL (22.2±5.3mm). There was no difference in SL between resident and anadromous individuals during age at first migration, suggesting that size may not be the mechanism for divergence of alternative life history styles.     

Salinity-linked growth in anguillid eels and the paradox of temperate-zone catadromy

Temperate-zone anguillid eels use both saline (marine or brackish) and fresh waters during their continental phase, but use of fresh waters is paradoxical because on average these fishes grow more rapidly in saline than in fresh waters. Based on data from anguillid eels whose habitat-residency histories had been determined by Sr:Ca otolithometry, superiority of growth rates in saline water is much greater in American eels Anguilla rostrata in north-eastern North America (mean saline:fresh growth rate ratio 2·07) than in European Anguilla anguilla , Japanese Anguilla japonica and shortfinned Anguilla australis eels (range of mean ratios 1·12–1·14). Data from A. rostrata in the Hudson Estuary, U.S.A., and Prince Edward Island, Canada, were used to test adaptive explanations of catadromous migrations. The hypothesis that lower mortality in fresh water offsets faster growth in saline water was not supported because loss (mortality + emigration ) rates did not vary between saline and fresh zones of the Hudson Estuary. Hypotheses that anguillid eels move to fresh water to escape from larger anguillid eels in saline water or to evaluate habitat quality were not supported by size and age distributions. Catadromy in temperate-zone anguillid eels increases the diversity of occupied habitats and therefore lowers fitness variance caused by environmental fluctuations. Catadromy in temperate-zone anguillid eels could be due to natural selection for maximum geometric mean fitness which is sensitive to fitness variance. Temperate-zone catadromy might also be maladaptive, at least in local areas, due to shifts over time in selective pressures or to inability of panmictic genetic systems to adapt to local conditions.     

Is the marginal otolith increment width a reliable recent growth index for larval and juvenile herring?

Marginal otolith increment width analysis was performed on field‐collected larval and juvenile spring‐spawned herring Clupea harengus that experienced variable feeding conditions and high temperatures that were above the optimum for growth. Although drastic zooplankton biomass reduction had a significant effect on increment width, a delay of a few days in the otolith response was observed. More importantly, a very clear, positive temperature effect on marginal increment width was demonstrated in fish characterized by temperature independent somatic growth. These results indicate that under natural conditions it may be impossible to distinguish increment width changes related to variation in feeding conditions from changes caused by temperature fluctuations. Therefore, it was concluded that marginal otolith increment width analysis could not be used as a recent growth index ( I _G) for herring larvae and juveniles exposed to drastic temperature fluctuations. The implication of these results is significant not only for the use of marginal increments as a recent growth index, but also if growth rate backcalculation is to be used as a research method.     

Habitat selection as a source of inter‐specific differences in recruitment of two diadromous fish species

• 1 For aquatic species with highly dispersive offspring, the addition of new individuals into an area (recruitment) is a key process in determining local population size so understanding the causes of recruitment variability is critical. While three general causative mechanisms have been identified (the supply of individuals, habitat selection and mortality), we have a limited understanding of how variation in each is generated, and the consequences this may have for the spatial and temporal distribution of recruits.
• 2 We examined whether active habitat selection during settlement could be the cause of variability in populations of two diadromous fish species using a field survey and laboratory‐based choice experiments. If larval behaviour is important, we predicted there would be inter‐specific differences in abundance between sites during the survey, and that larvae would prefer water collected from sites with higher conspecific abundances during the experiments.
• 3 During the field survey, significant differences were detected between two rivers (the Cumberland and Grey), with one species (Galaxias maculatus) found in higher abundances at one site (the Cumberland River) while comparable numbers of a closely related species (Galaxias brevipinnis) were caught at both sites. Laboratory choice experiments were conducted to determine whether larval preferences during settlement could be the cause of these differences. G. maculatus larvae showed a preference for freshwater over saltwater, indicating that the fish may be responding to reduced salinities around river mouths during settlement. The results of a second experiment were consistent with the notion that larval preferences could be the mechanism driving differences in the populations of the two rivers, with G. maculatus preferring water collected from the Cumberland River while G. brevipinnis did not prefer water from either river.
• 4 These results demonstrate that active habitat selection may be important in establishing spatial patterns of larvae at settlement, and that multiple cues are likely to be involved. This study also demonstrates that the behaviours exhibited by individuals can strongly influence the structure and dynamics of populations of aquatic species with complex life cycles.

     

Bigger? Fatter? Or is faster growth better? Considerations on condition in larval and juvenile coral-reef fish

Abstract Analyses of condition data are conspicuous by their paucity in the extensive tropical reef-fish literature. Researchers typically quantify abundance at settlement, with little regard for the demonstrably variable quality of newly settled fishes. Condition may be functionally classified by indices of growth (e.g. the RNA-DNA index or peripheral growth increments of the otolith), starvation (e.g. height of midgut mucosal cells), storage (e.g. lipid content), or morphometry (e.g. dry weight/length’), all of which are variably correlated with each other. At present all indices are species-, stage-, technique- and therefore often investigation-specific, as laboratory-reared larvae for calibrating field-collected condition indices are often specific to the rearing procedure. RNA indices are particularly appropriate for estimating larval condition. In pelagic juveniles, or in recently settled juveniles, the width of peripheral growth increments of the otolith estimates average growth rate in length or dry weight during the previous few days, which discerns increasing from decreasing condition. Increment width changes in otoliths are particularly responsive to starvation events, and are correlated with RNA indices. Growth indices have great potential for determining which individuals were growing faster, thereby reducing their pelagic duration, and thus increasing their survival potential. The recent debate regarding whether bigger larvae have better survival could be re-addressed, by determining if larvae with faster growth indices have relatively enhanced survival.     

Comparison between the life cycles of two Soleidae, the common sole, Solea solea, and the thickback sole, Microchirus variegatus, in the Bay of Biscay (France)

The life history of two Soleidae, the common sole, Solea solea, and the thickback sole, Microchirus variegatus, were compared in the Bay of Biscay in an attempt to set out factors which could explain settlement styles known to be different between juveniles of each species. Common sole juveniles had been shown to depend on coastal and estuarine nurseries, and the thickback sole to develop in open-sea nurseries, although the spawning grounds and spawning season of both species overlapped (offshore and at springtime, respectively). For this study, data on adult, juvenile and larva distributions were obtained from cruises carried out in the Bay of Biscay during the last decade. In addition, growth rate of larvae and planktonic interval duration, estimated by means of otolith increment analysis, were compared, as well as literature-derived information on behaviour of larvae. By comparing larval features, it appeared that thickback sole did not obtain the advantage of a slightly longer pelagic life span for an increased dispersal, due to an early shift to benthic behaviour. Other evidence was given by more specifically stated distributions that the further and deeper offshore spawning of M. variegatus, compared to S. solea, was a likely key-factor of the juvenile settlement process. This could explain why the thickback sole, contrary to the common sole, never reach coastal areas and thus settle offshore, in waters deeper than 30 m.     

Seasonal and long-term changes in biometrics of eel larvae: a possible relationship between recruitment variation and North Atlantic ecosystem productivity

The European eel larval life has two stages: the oceanic leptocephalus, in which growth is dependent on food supply, and the metamorphosis glass eel stage, in which feeding and growth are stopped until immigration in estuarine waters. Data from fisheries and experimental surveys in European estuaries at different periods between the 1930s and the 1990s showed a simultaneous decrease of mean length and abundance level. A monthly survey at the Vilaine estuary (Bay of Biscay) during 2 years provided a periodical model of seasonal variation in length and weight of newly recruited glass eels. Otolith microstructure was used to back-calculate larval stage durations and the timing of transatlantic migration. Glass eels entering the estuary in autumn are longer and more numerous than those arriving in summer. They migrated across the ocean during spring and summer, seasons when plankton production is likely to reach maximum levels. The proposed hypothesis regarding the determinism of recruitment in the eel, on the seasonal and on the yearly range, is that leptocephalus growth and glass eel survival are partly dependent on the plankton production at the oceanic scale. Eel recruitment dynamics could provide an integrated figure of large-scale environment fluctuations.