Contrasting patterns of larval mortality in two sympatric riverine fish species: A test of the critical period hypothesis

Understanding the causal mechanisms that determine recruitment success is critical to the effective conservation of wild fish populations. Although recruitment strength is likely determined during early life when mortality is greatest, few studies have documented age-specific mortality rates for fish during this period. We investigated age-specific mortality of individual cohorts of two species of riverine fish from yolksac larvae to juveniles, assaying for the presence of a "critical period": A time when mortality is unusually high. Early life stages of carp gudgeons (Hypseleotris spp.) and unspecked hardyhead (Craterocephalus stercusmuscarum fulvus)-two fishes that differ in fecundity, egg size and overlap between endogenous and exogenous feeding-were collected every second day for four months. We fitted survivorship curves to 22 carp gudgeon and 15 unspecked hardyhead four-day cohorts and tested several mortality functions. Mortality rates declined with age for carp gudgeon, with mean instantaneous mortality rates (-Z) ranging from 1.40-0.03. In contrast, mortality rates for unspecked hardyhead were constant across the larval period, with a mean -Z of 0.15. There was strong evidence of a critical period for carp gudgeon larvae from hatch until 6 days old, and no evidence of a critical period for unspecked hardyhead. Total larval mortality for carp gudgeon and unspecked hardyhead up to 24 days of age was estimated to be 97.8 and 94.3%, respectively. We hypothesise that life history strategy may play an important role in shaping overall mortality and the pattern of mortality during early life in these two fishes

Aquatic Survey of Purda Billabong, Near Wentworth NSW

"April 2003".Project Number: Purda Billabong aquatic fauna survey - M/BUS/70.MDFRC item.Purda Billabong, otherwise known as 'Pink Lake', is an oxbow wetland located on 'Moorna Station' (Walsh family) approximately 30 km west of the township of Wentworth NSW. The wetland is approximately 174 hectares in size and is connected to the Murray River via a small feeder channel off Frenchman's Creek. Due to the influence of Lock 9, the majority of the surface area of the wetland is permanently inundated. The NSW Murray Wetlands Working Group, in conjunction with the Walsh family of 'Moorna Station', are investigating options to restore a more natural hydrological regime to the wetland by constructing a regulating device on the inlet channel that links the wetland to the Murray River. An aquatic fauna survey was conducted in November 2002 to obtain baseline information on the current diversity of biota within the wetland. The water quality of Purda Billabong was good with levels similar to those recorded in a nearby wetland known as Thegoa Lagoon (except for turbidity) and the Murray River downstream of Lock 8. Dissolved oxygen and pH levels were similar between sites and sampling occasions whereas turbidity, water temperature and electrical conductivity levels varied. Electrical conductivity was seen to increase from the inlet to the furthest site from the inlet due to evaporation and the diluting effect of water entering the wetland from the Murray River. Nutrient levels in Purda Billabong vaired little between sites and are were found to be substantially lower than levels measured in Thegoa Lagoon. Phytoplankton biomass (chlorophyll concentrations) varied little between sites within the wetland and were considered low in comparison to concentrations recorded in the Murray River upstream of the Darling River junction at a similar time of year. A total of 4584 individual microinvertebrates representing 10 taxa were collected in Purda Billabong, all of which were considered to be widespread in South Eastern Australia. The variability in taxa richness and abundance encounted between sites was considered typical of the spatial and temporal variability of open water microinvertebrate communities. Macroinvertebrates were sampled in the wetland with artificial substrates and a sweep net. A total of 34,930 macroinvertebrates representing 25 taxa were collected during the study. Of these 77% were captured in 5 artificial substrates (29,928 individuals and 23 taxa) compared to 23% (7,982 individuals and 14 families) in the single 10 m edge sweep sample. Artificial substrates were dominated by benthic taxa such as the dipterans (mostly chironomids) while the sweep net sample was dominated by more mobile pelagic taxa such as hemipterans (mostly corixids). SIGNAL (Stream Invertebrate Grade Number – Average Level) scores were used to calculate the average sensitivity of macroinvertebrate taxa at each site. SIGNAL scores ranged from 3 to 4 out of 10 suggesting that only relatively tolerant individuals were present in Purda Billabong. The low scores are not unexpected for lowland river environments where sensitive taxa are typically absent. A total of 1105 individual fish representing 11 taxa (4 alien and 7 native species) were collected from Purda Billabong. All of the species sampled were considered to be common in lowland environments. The Carp Gudgeon complex and Australian Smelt were the most abundant native species contribution to 48% and 29% of the total catch respectively. Carp were the most common alien species contribution to 10% of the total catch. The diversity and abundance of aquatic biota from Purda Billabong is similar to that of Thegoa Lagoon near Wentworth NSW. By reinstating the natural hydrological regime, the productivity of the Purda Billabong should increase in the short term and an improvement in the biodiversity values of the wetland in the long term. The following recommendations are based on results from the fauna survey. - A regulatory structure should be installed at the junction of the inlet channel with Frenchman's Creek to initiate a drying phase within the wetland. - The length of time, frequency and magnitude of the drying phase should be linked to the natural hydrological regime prior to river regulation. - Groundwater testing and monitoring be conducted to determine the potential for groundwater to affect the wetland. - The drying phase should be initiated from the current level of the billabong to minimize habitat loss for invertebrates and fish. - Stock access to the drying sediment should be restricted to maintain water quality and invertebrate egg masses, particularly at the shallow eastern end of the billabong where the majority of the wetland sediments will be exposed. - Continued monitoring of the aquatic macroinvertebrate community during drying and wetting cycles

Individual cohort survivorship curves for carp gudgeon larvae in the Lindsay River between October 2005 and February 2006.

<p>Data log_e (x+1) transformed. Black dotted line  =  Weibull function (non-constant mortality model), black solid line  =  asymptote function (non-constant mortality model), and grey solid line  =  linear function (constant mortality model). Age class (days) represent 2 day groupings of larvae (e.g. 2 = 1-2 day old larvae, 4 = 3–4 day old larvae etc).</p

Map of the study area; Lindsay River, Victoria, Australia.

<p>Map of the study area; Lindsay River, Victoria, Australia.</p

Individual cohort survivorship curves for unspecked hardyhead larvae in the Lindsay River between October 2005 and February 2006.

<p>Data log_e (x+1) transformed. Black dotted line  =  Weibull function (non-constant mortality model), black solid line  =  asymptote function (non-constant mortality model), and grey solid line  =  linear function (constant mortality model). Age class (days) represent 2 day groupings of larvae (e.g. 2 = 1–2 day old larvae, 4 = 3–4 day old larvae etc).</p

AICc results for the alternative models of mortality during the larval phase of carp gudgeon and unspecked hardyhead for each 4d cohort; linear (constant mortality (Z)), asymptote (non-constant mortality (Z)) and Weibull (non-constant mortality (Z)) functions.

<p>Bold = model of best fit. CP  =  critical period, where Y = yes, N = no,? =  could not be determined.</p><p>AICc results for the alternative models of mortality during the larval phase of carp gudgeon and unspecked hardyhead for each 4d cohort; linear (constant mortality (Z)), asymptote (non-constant mortality (Z)) and Weibull (non-constant mortality (Z)) functions.</p

Mean (±SE) cohort; instantaneous mortality rates (-<i>Z</i>), daily mortality rates (M_daily) and cumulative survival (%) of carp gudgeon and unspecked hardyhead larvae.

<p>Age class (days) represent 2 day groupings of larvae (e.g. 2 = 1–2 day old larvae, 4 = 3–4 day old larvae etc).</p

Life history traits of carp gudgeon and unspecked hardyhead.

<p>Life history traits of carp gudgeon and unspecked hardyhead.</p

Monthly frequency box-plots of age (days) of each key developmental stage for a) carp gudgeon and b) unspecked hardyhead.

<p>October born larvae (white), November born larvae (white-striped), December born larvae (grey) and January born larvae (grey-striped).</p

Riverscape recruitment: a conceptual synthesis of drivers of fish recruitment in rivers

Most fish recruitment models consider only one or a few drivers in isolation, rarely include species’ traits, and have limited relevance to riverine environments. Despite their diversity, riverine fishes share sufficient characteristics that prediction of recruitment should be possible. Here we synthesize the essential components of fish recruitment hypotheses and the key features of rivers to develop a model that predicts relative recruitment strength, for all fishes, in rivers under all flow conditions. The model proposes that interactions between flow and physical complexity will create locations in rivers, at mesoscales, where energy and nutrients are enriched. The resultant production of small prey will be concentrated and prey and fish larvae located (through dispersal or retention) so that the larvae can feed, grow, and recruit. Our synthesis explains how flow and physical complexity affect fish recruitment and provides a conceptual basis to better conserve and manage riverine fishes globally.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author