Fish and flows: Abiotic drivers influence the recruitment response of a freshwater fish community throughout a regulated lotic system of the Murray-Darling Basin,Australia

1. Fish assemblages in dryland rivers have life-history strategies that have evolved in response to environmental conditions and triggers, particularly water temperatures and flow regimes. The regulation of rivers through the construction of dams, weirs and other water diversion structures has altered natural flow regimes and the associated ecological processes of river systems worldwide.
2. Over a 3-year period, using standardized fish sampling and daily otolith ageing, the recruitment of eight freshwater fish species was monitored in response to various abiotic drivers, including hydrology and water temperatures, throughout the Macquarie River, a large regulated river system of the Australian Murray-Darling Basin.
3. A data-driven statistical classification system is provided that groups species into reproductive guilds, based on their recruitment response to hydrology and water temperature, specifically designed for use in environmental flow management.
4. The eight species were grouped into three distinct reproductive guilds that showed similar recruitment responses to the abiotic drivers. Murray-Darling rainbowfish, Murray cod, and eel-tailed catfish were considered as a single guild, characterized by seasonal recruitment during a relatively narrow thermal window under low to moderate stable discharges. The second group included common carp, bony herring, and golden perch, which recruited primarily in association with larger flow events. Un-specked hardyhead and Australian smelt formed a more differentiated guild, recruiting over a broad range of temperatures and discharges. Limitations associated with using a reproductive guild approach to simplify water management are discussed.
5. This study highlighted important relationships among hydrology, water temperatures, and successful recruitment that can, in turn, be used to inform development of adaptive flow management plans and effective use of environmental water for the conservation management of native fish communities. Important considerations in the design of studies that aim to examine relationships between recruitment and abiotic drivers are also discussed.

     

Spatial and temporal variation in otolith chemistry and its relationship with water chemistry: Stock discrimination of Sperata aor

Long‐whiskered catfish, Sperata aor (Hamilton 1822), is commercially important in food, ornamental and sport fisheries. The fish is mainly caught from the wild populations because its aquaculture practices are not commercialised. Inland fishery in the Ganga basin is mostly unorganised; hence, no published report is available on the trend of S. aor production from the selected habitat. In India, S. aor has been categorised vulnerable mainly due to natural and anthropogenic threats. Otolith chemistry shows variation with changing physico‐chemical conditions of the fish habitat. Therefore, the present study was conducted with the objective to analyse spatio‐temporal variations in water chemistry in relation to environmental factors; relationship between water and otolith chemistry; and spatio‐temporal variations in otolith chemistry to discriminate the stocks of S. aor inhabiting the River Ganga. Most of the element: Ca ratios in water samples did not show significant correlations with environmental factors, viz. temperature and conductivity. Only few element: Ca concentrations in otoliths were positively correlated to their corresponding ratios in the ambient water. In the selected study area, the S. aor populations were discriminated into four stocks possibly because of heterogeneous water chemistry at the sampling sites, and physical barriers. In the present study, otolith chemistry showed relatively low temporal variability as compared to spatial variability; thus, the classification accuracy of individuals to their original populations remained consistent over the selected time period. The findings could be useful in devising scientifically sound management strategies and/or any conservation plans for the vulnerable S. aor populations inhabiting the River Ganga.     

Using otolith microchemical analysis to investigate the importance of brackish bays for pike (Esox lucius Linnaeus, 1758) reproduction in the southern Baltic Sea

The brackish Baltic Sea is inhabited by several primary freshwater fish species. The pike (Esox lucius) is the largest of these species and acts as the top predator in the marine food web of the Baltic Sea. Prior studies have shown the importance of brackish bays as spawning grounds for the Baltic Sea pike. To identify the spawning habitat of the pike population around the island of Rügen (southern Baltic Sea)—either freshwater streams flowing into the Baltic or brackish lagoons of the Baltic—we used the ICPMS method to quantify the strontium to calcium (Sr:Ca) ratios in sagittal otoliths of 79 adult pike. In total, only six per cent (n = 5) of the analysed Baltic Sea individuals were classified as being of freshwater origin. Ninety‐four per cent (n = 74) of analysed pike were classified as having hatched in brackish water. Compared to previous studies on Baltic Sea pike, our sample shows a high proportion of pike that is of brackish origin: this highlights the adaptive capacity of this species with regard to its ability to reproduce in different salinities. Our data highlight the importance of brackish bays for pike reproduction in the southern Baltic Sea, even if pike live on the upper limit of their salinity tolerance in this area. This is an important finding considering decreases in commercial catches in the last 30 years indicating a population decline supposedly related to recruitment failures.     

The effect of lake morphometry on thermal habitat use and growth in Arctic charr populations: implications for understanding climate‐change impacts

Oxygen stable isotope temperature reconstruction methods were used to estimate mean experienced summer temperatures from growth zones within individual Arctic charr otoliths sampled from lakes with contrasting morphologies but proximate locations. For either lake, otolith‐estimated temperatures were not significantly related to back‐calculated growth. Fish in the smaller lake evidenced an increase in growth with age related to increasing use of cooler thermal habitats, with the use of thermal habitat possibly governed by predation risks. No relationships between age, growth or temperature were observed in the larger lake. Significant negative effects on back‐calculated growth were observed due to increasing air temperatures in the smaller and shallower lake, possibly owing to warmer surface and littoral waters and a limited amount of preferred cool‐water habitat. A similar relationship was not observed in the larger and deeper lake and indicated that resident Arctic charr were not as vulnerable to the impacts of temperature warming, possibly because of better behavioural thermoregulation opportunities in the cooler, deeper lake. Results provide evidence for differing climate‐influenced growth outcomes among proximately located populations, with outcomes likely to depend on the differences among habitats, including lake size and morphometry which may act to influence fish densities in available preferred thermal habitats.     

An evaluation of different structures to age freshwater fish from a northeastern US river

Abstract  Largemouth bass, Micropterus salmoides (Lacepède), smallmouth bass, Micropterus dolomieu Lacepède, and yellow perch, Perca flavescens (Mitchill), were collected from the Hudson River, New York, USA, to compare the precision of age estimates derived from scales and otoliths. Similar procedures were used to compare otolith and spine ages from brown bullhead, Ameiurus nebulosus (Lesueur). Overall percent agreement between readers ranged from 91% to 98% for otoliths compared with 38% to 67% for scales and spines. Disagreement rates associated with scales and spines increased as fish grew older. Average percent error between readers was about an order of magnitude higher for scales and spines than for otoliths. Ages estimated from scales and spines progressively decreased as age increased based on otolith examination. The use of scales and spines to age largemouth bass, smallmouth bass, yellow perch and brown bullheads from the northeastern US was less precise and will likely lead to underestimation of age of larger and older fish.     

Energy Metabolism of the Cattle Egret

Ages were estimated for the tropical sparid, Acanthopagrus berda, caught in northern KwaZulu-Natal estuaries. Whole otoliths were used in the age determination. Age estimates were validated by marginal increment analysis and oxytetracycline labelling, which indicated that opaque deposition occurs primarily from September to November each year. The reproducibility of age estimates was described by a coefficient of variation of 10%. The von Bertalanffy growth curve was found to best describe the growth of A. berda. The parameters of the von Bertalanffy growth curve indicated that A. berda in northern KwaZulu-Natal is a slowgrowing species, capable of reaching at least 16 years of age. Longevity of the species, coupled with sex change, late maturation and estuarine dependency, give cause for concern for the continued sustainable utilization of this species.     

Taxonomic and geographic influences on fish otolith microchemistry

Fish otoliths are comprised primarily of CaCO₃ and grow throughout an individual's lifetime. The chemical composition of otoliths is often a distinctive characteristic of the populations that live in discrete areas, and as a result, it has been used for population classification studies, supporting ecological and fisheries research. However, the deposition of chemical elements in the otolith is influenced by both physiological and environmental factors. We review observed trends in otolith elemental composition and then test the taxonomic and geographical patterns, using marine species in European waters. anova comparisons and multivariate analyses revealed strong taxonomic signals in species inhabiting the same region. Variations in Sr, Mg, Mn and Ba concentrations were most often species specific; for example, multivariate analyses showed separation of cod (Gadus morhua) and herring (Clupea harengus) based primarily on Mg and Sr concentrations, while Mn and Ba concentrations separated bluemouth (Helicolenus dactylopterus) and sole (Solea solea). The relative contributions of taxonomic and regional factors varied between elements. For cod and herring, for example, species‐specific differences explain 75% of the variation for Mn, but only 50% of the variation in Sr. Although there are significant regional differences within a single species on both a restricted and extensive geographic scale, these regional patterns are not the same for each species. As the amount of otolith composition data increases, representing more species and regions, as well as longer time series, further analyses can provide a deeper insight into the predictability of using otolith data in fisheries ecology.     

Assigning origins in a potentially mixed‐stock recreational sea trout ( Salmo trutta) fishery

The anadromous, or sea‐going, life history form of brown trout, or sea trout ( Salmo trutta), may lead to potential mixing of populations while foraging at sea. In this article, we assess the potential that multiple populations are using common semi‐enclosed estuaries and quantify the potential levels of straying (i.e. dispersal) of foreign‐produced individuals into populations by using otolith chemical signatures as natural ‘tags’. To do so, we created a database of juvenile fish otolith chemistry (a marker of freshwater production) from four rivers and compared the chemistry of harvested fish in two estuaries important to anglers, the Renews River and Chance Cove Brook, to the database. A discriminant function analysis revealed significant differences in the otolith chemistry of juvenile fish inhabiting the four rivers with a 97% cross‐validated accuracy when classifying individual juveniles to their natal river, indicating our baseline was robust. When assigning adults caught over 3 years (2007–2009) in the recreational fishery in the Renews River estuary, it was determined that over 95% of the fish caught each year originated from Renews River. In contrast, harvested fish in Chance Cove during 2009 were disproportionately comprised of fish produced in Renews River, suggesting the potential for source‐sink population dynamics in Newfoundland. Taken as a whole, these results indicate limited population mixing in nearshore estuaries of this region, but also highlight the potential for some populations to subsidise the harvest by anglers in different areas.     

Emergence of larval yellow perch, Perca flavescens, in South Dakota lakes: potential implications for recruitment

Abstract  Temporal patterns in length frequency distributions and hatch dates were described for larval yellow perch, Perca flavescens (Mitchill), captured in surface ichthyoplankton trawls from late April to mid-June 2000 to 2002 in six South Dakota, USA lakes. Fewer than 15 larval yellow perch were collected in four of six lakes during 2002, suggesting that in some cases factors prior to, during or immediately after hatching likely play a critical role in the perch recruitment process. When larval yellow perch were encountered in larger numbers, temporal trends in total length (TL) frequencies indicated that only a single cohort was produced annually in each lake. Most yellow perch in these lakes hatched between 29 April and 17 May, and most hatching occurred during 5–11 days each year. Larval TL was not related to hatch date. The apparent prevalence of relatively short hatch periods in these yellow perch populations probably increases the risk of catastrophic losses resulting from periods of poor environmental conditions.