Life history characteristics of the fluvial shrimp Macrobrachium yui (Holthuis, 1950) (Decapoda, Palaemonidae) spawning in the cave streams in northern Laos

This study aims to understand the reproductive and larval characteristics and the migratory patterns of the shrimpMacrobrachium yui(Holthuis, 1950). We conducted monthly samplings of the adult, juvenile and larval shrimp at fixed points in the Xuang River system in northern Laos. We determined the seasonal changes in size frequency distribution, difference in size among sampling sites, occurrence of drifting larvae, and physiological condition of the shrimp. The size distributions ofM. yuidiffered significantly throughout the year among sections of the river system: small-sized juveniles occurred in the Xuang River, various size classes in the Pho Stream, and larger adults in the Bokengsakang cave stream. The gonadosomatic index of females is the highest in the Bokengsakang cave stream and the gonadosomatic index of males is the highest in the Pho Stream. Drifting larvae that had already completed morphological development for settlement on the substrate occurred only in the Bokengsakang cave stream, although the hatched larvae have a free-swimming larval period. The overall results indicate thatM. yuifemales spawn in the Bokengsakang cave stream after mating with males during or before migration to the cave stream, and the hatched larvae remain in the cave stream until settling on the bottom. The life history migration ofM. yuiis attributed to the early stages of the life cycle, depending strongly on the cave stream.

Dispersal and retention of larval fish in a potential nursery habitat of a large temperate river: an experimental study

Little information is available on governing factors of larval fish dispersal in natural river systems. Therefore, we aimed to describe dispersal and retention of marked larval nase carp, Chondrostoma nasus , along a shoreline nursery habitat of the River Danube. Based on a three-dimensional hydrodynamic model, we analyzed the influence of the hydraulic conditions on larval dispersal. We also related observed larval pathways to numerical particle tracing. Clear differences in the temporal drift pattern were due to significant differences in the hydrodynamic characteristics of the release stations. Some larvae remained in the study reach, most upstream of the release point. These were significantly larger than drifting larvae. We conclude that larval dispersal has an active component and that dispersal and retention patterns are dependent on the habitat structure and hydrodynamic characteristics of the releasing points. This emphasizes the importance of links between the location of spawning sites within the river and variation in flow during early development, the combination of which may contribute to successful recruitment of fluvial fish species.

Collection and distribution of the early life stages of the Murray cod (Maccullochella peelii peelii) in a regulated river

The Murray cod (Maccullochella peelii peelii) is a large fish species keenly sought by anglers. However, this species has declined in distribution and abundance and is now listed nationally as vulnerable. This study was undertaken in the Ovens and Murray rivers, to collect larvae and age-0 Murray cod and determine the distribution of larval Murray cod around the mid-Murray River irrigation storage of Lake Mulwala. Murray cod larvae were collected from 17 of 18 sites: main channels and flowing anabranch channels of regulated and unregulated rivers, sites upstream and downstream of the lake, in the upper and lower reaches of the lake, and in the outflowing Yarrawonga irrigation channel. Larval Murray cod were collected only by methods that sampled drift in flowing waters. Age-0 Murray cod were collected by electrofishing in the main river, but not in off-channel waters, suggesting that cod are likely to settle into habitats in the main channel at a post-larval stage. The widespread occurrence of drifting larvae suggests that this species may be subject to previously unrecognised threats as they pass through hydro-electric power stations or become stranded in anabranch and irrigation channels. Results of this study are likely to be applicable to other species with drifting larval stages, and are relevant to other locations in the Murray–Darling Basin.

HIGH-DOSAGE TREATMENT OF A QUEBEC STREAM WITH BACILLUS THURINGIENSIS SEROVAR. ISRAELENSIS: EFFICACY AGAINST BLACK FLY LARVAE (DIPTERA: SIMULIIDAE) AND IMPACT ON NON-TARGET INSECTS

A typical lake outlet of the Canadian Shield was treated for 15 min with a high dose (5.28 g/L s−1 of discharge) of Teknar®, a commercial formulation of Bacillus thuringiensis serovar. israelensis. Efficacy on Simuliidae larvae and impact on non-target aquatic insects of this stream were monitored using drift nets, counting plates, and artificial turf substrates along a 1000-m section downstream of the site of application. Compared with a 4-day pre-treatment average for 12-h sampling periods, drift of Simuliidae increased from 64 to 92 ×, with shorter peaks of 133–184 ×, 2–6 h after treatment. There was no evident drift increase in larvae of Ephemeroptera, Plecoptera, Trichoptera, Chironomidae, or dipterous pupae, but larvae of Blephariceridae (Diptera) were severely affected as their drift was increased by up to 50 × and remained high for 3 days. After 30 h the mortality of Simuliidae on counting plates ranged from 95 to 82% in the first 300 m, with detachment rates of 78.5–46.5%. Densities of non-target insect larvae were not reduced on the artificial substrates, except for 2 genera of Chironomidae (Eukiefferella and Polypedilum) which were reduced 26 to 39% of their original density. Drifting larvae of 1 chironomid genus (Phaenopsectra) also showed symptoms of toxemia by B.t.i. The main impact of the treatment was thus seen in 2 Nematocera families (Chironomidae and Blephariceridae) which were mainly exposed to B.t.i. sedimented on the bottom of the stream or attached to periphyton growing on rocks.