Diet of Murray cod (Maccullochella peelii peelii) (Mitchell) larvae in an Australian lowland river in low flow and high flow years

Abstract

Researchers have hypothesised that influxes of pelagic zooplankton to river channels after floods and high flows are necessary for strong recruitment of some native fish species, including Murray cod (Maccullochella peelii peelii) (Mitchell), in the Murray-Darling river system, Australia. This study investigated the composition of the diet and gut fullness of drifting Murray cod larvae weekly during two spawning seasons with contrasting flows, to determine if pelagic zooplankton comprised a greater proportion of the gut contents and guts were fuller in a high flow (2000) than in a low flow (2001) year. Gut fullness and yolk levels of 267 larvae were ranked, and prey identified to family level. Approximately 40 and 70% of individuals had been feeding in 2000 and 2001, respectively. Gut fullness increased with declining yolk reserves. Larvae in both the years had an almost exclusively benthic diet, irrespective of the flow conditions at the time. Substantial inundation of dry ground in 2000, albeit restricted to in-channel benches, anastomosing channels and oxbow lakes, did not lead to an influx of pelagic, floodplain-derived zooplankton subsequently exploited by Murray cod larvae. These results have the implications for the management of regulated temperate lowland rivers: high flows cannot automatically be assumed to be beneficial for the fish larvae of all species and their food resources, and caution should be exercised with the timing of flow releases.

Fish recruitment in rivers with modified discharge depends on the interacting effects of flow and thermal regimes

“…Reduced abundances of M. ambigua recruits in regulated rivers are likely to be due to the effect of flow on the composition and abundance of invertebrates that serve as important prey sources for juvenile fish (Balcombe et al ., ). In the Weir River, frequent periods of low or no flow are likely to promote juvenile recruitment due to reduced scouring of prey by higher flow, the increased production of autochthonous energy sources and biomass of invertebrates linked with increased residence time of water, coupled with decreased area and connectivity of remaining pools (Baranyi et al ., ; Bunn et al ., ; Zeug & Winemiller, ; Kaminskas & Humphries, ). In conjunction, abundances of juvenile recruits are positively linked with slow velocity and shallow habitats during stable low flows that serve as key nursery habitats (Freeman et al ., ; Pease et al ., ).…”

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

“…2; Table 2; Schiemer et al 2001). High concentrations of prey may be less important for other fish that have large stores of yolk at hatch and thus their own intrinsic source of nutrient enrichment (Winemiller and Rose 1993;King 2005;Kaminskas and Humphries 2009), allowing them to effectively bypass the larval stage (sensu Balon 1999). Therefore, because fishes differ enormously in their life history and other species traits, the nature of, and requirements associated with, enrichment, concentration, and retention-dispersal processes will vary among species.…”

“…By contrast, the larvae of equilibrium species typically have large amounts of yolk, which they consume over many days; they are large, well developed, and swim well when first feeding exogenously. The larvae of Murray cod (Maccullochella peelii), for example, are 8-10 mm and typically 7-10 days old when first feeding (Kaminskas and Humphries 2009) and have critical swimming speeds at this time of 15-30 cm·s -1 (Kopf et al 2014). First-feeding equilibrium species typically feed on larger meiofauna and early instars of insects, such as benthic cladocerans and chironomids (King 2005;Kaminskas and Humphries 2009;Sauvanet et al 2013).…”

“…It will only be once the yolk is used up that suitable concentrations of prey will be required. But by then, many of the food-related challenges associated with recruitment have been overcome, and the comparatively large juveniles, with wide mouth gapes, have access to a large range of invertebrates as prey (King 2005;Kaminskas and Humphries 2009). For these species, mortality may be highest at the juvenile stage, when other demands such as finding a territory, competition for resources, and predator avoidance become critical (Houde 1994).…”

Ontogenetic behaviour and swimming ability of the endangered eastern freshwater cod, Maccullochella ikei, with notes on growth and development

“…Characteristically, when plankton were introduced into the aquaria, M. ikei larvae would rapidly ascend through the water column ingesting prey as they went, before drifting or swimming back towards the bottom of the tank and then repeating the process until the plankton were depleted or they were full. The pelagic feeding strategy suggested for M. ikei differs somewhat from that hypothesised for the closely related M. peelii, which is reported to be primarily a benthic feeder during the initial stages of dispersal in the wild (King 2005;Kaminskas & Humphries 2009). However, the different feeding strategies may relate to differences in the turbidity of the waters where the two species occur.…”