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The blind cavefish, Milyeringa veritas, inhabits an anchialine system, effectively a groundwater estuary, in which salinity varies between fresh and seawater at different locations and depths. Owing to the inaccessible habitat and the threatened status of the cavefish it is hard to obtain the biological information needed for their management. This paper explores the utility of otolith Sr:Ca ratio in elucidating cavefish biology. The mean Sr:Ca ratio of the water inhabited by the cavefish is correlated with both the TDS (total dissolved solids) of the habitat and with the Sr:Ca ratio of the sagittal otolith of the cavefish inhabiting that site. Mean values of Sr:Ca in otoliths suggest some cavefish inhabit sea or brackish waters while others remain in freshwater. Some individuals appear to move between waters of very different TDS at various stages but there is no consistency in the direction or apparent TDS range of the water bodies inhabited which indicates that the cavefish utilise the different water bodies opportunistically. Residual analyses indicate clear and routine changes in the TDS of the water occupied at various phases of growth, irrespective of the TDS at which the cavefish were sampled. Annular markings are present in some otoliths but they cannot be related to likely periodicities in the subterranean environment.

The depth distribution of the common bully, Gobiomorphus cotidianus, a small benthic forage fish, was measured by trapping at set depths from 0–70 m in three large oligotrophic lakes, including one where inorganic sediment from a glacially-fed river produces turbid conditions. Bullies occurred at all depths from 0.5–70 m in the clear lakes, but none were present below 25 m in the turbid lake. Two groups of bullies were present in the clear lakes; a high-density, littoral stock at depths of 0.5–25 m, and a low-density, profundal stock at depths of 30–70 m. These groups were further distinguished by differing buoyancy requirements and feeding habits. The swimbladders of littoral bullies contained gas, but those of the profundal bullies, which fed more than littoral bullies by both day and night, did not. The variation in mean CPUE with depth within the littoral zone was not related to water temperature, oxygen concentration, or conductivity. Nor was it related to a reduction in light levels or to reduced water transparency caused by increased turbidity. It may therefore be controlled by biotic factors. The absence of a profundal stock below the littoral zone in the turbid lake indicates that the settlement of fines from turbid inputs may affect bully abundance in deeper waters. As conventional measures of the abundance of benthic fish in lakes are often restricted to littoral habitats, and do not reflect changes in abundance with depth, an index of overall abundance based on depth distribution was developed to allow comparisons between lakes.

The population structure, abundance, reproductive status, and feeding interrelationships of eleotrid fishes from two freshwater streams of the Caribbean coast of Costa Rica were investigated over 10 continuous months. Eleotris amblyopsis was abundant in both streams. Dormitator maculatus, Eleotris pisonis, and Gobiomorus dormitor were present in both streams but common only in the larger stream that contained dense growth of floating aquatic macrophytes. Dormitator maculatus is a detritivore and overlaps with the other three eleotrids, all of which had diets dominated by shrimp and fishes. Intraguild predation and cannibalism was documented with E. amblyopsis as the prey. Except for the benthic G. dormitor, eleotrids were associated with dense mats of floating macrophytes in the large stream, and leaf peaks in the small stream. Diet similarity among the three carnivorous eleotrids declined during the wet seasons, the period when their prey disperse at lower per-unit-area densities within the flooded forests that surround the streams. Analysis of population size distributions and gonadal maturation indicated that at least some reproduction occurs on a nearly year-round basis within these eleotrid populations. The spring dry period may be associated with migration of ripe eleotrids to estuarine habitats for spawning. Juvenile Eleotris are the dominant fishes in the ‘tismiche’, mass migrations of juvenile shrimp and fish larvae and juveniles that enter and ascend the estuary, especially during the summer wet season.

We compared the fish communities of two shallow lakes in the lower Waikato River basin, North Island, New Zealand, to determine the effects of elevated suspended solids (SS) and collapse of submerged macrophytes. Lake Waahi was turbid (20–40 g m-3 SS) and devoid of submerged macrophytes whereas Lake Whangape was clearer (5 g m-3 SS) and dominated by submerged macrophytes. The lakes had similar fish species richness and had nine major species in common; representing eight families including Anguillidae, Retropinnidae, Galaxiidae, Eleotridae, Mugilidae, Ictaluridae, Poeciliidae, and Cyprinidae (two species). The only major fish that was absent from Lake Waahi was a lacustrine form of the common smelt, Retropinna retropinna, which disappeared after the lake became turbid in the late 1970s. CPUE, condition, and size of most species in Lake Waahi were similar to, or greater than, those in Lake Whangape. Lake Whangape clearly exceeded Lake Waahi only for CPUE of two species. Within Lake Whangape two species displayed significantly greater condition, and one species greater size, in a turbid arm of the lake than in the main basin. Apart from lacustrine Retropinna retropinna, the fish in these lakes appear well adapted to cope with, or to avoid, the direct toxic effects of suspended and settleable solids on sensitive early developmental stages. In Lake Waahi loss of cover and food provided by submerged macrophytes appears to have been compensated for by increased turbidity and an associated increase in the biomass of the mysid, Tenagomysis chiltoni (a major prey item).

Oxyeleotris heterodon attains 483 mm standard length (1.83 kg) and Ophieleotris aporos 245 mm (0.171 kg). Both species feature significantly in the local fishery in lowlands. Ophieleotris aporos is approximately one hundred times more abundant than Ox. heterodon in catches and landings which were influenced by river flood conditions in both species. Oxyeleotris heterodon is predominantly piscivorous but also eats Macrobrachium spp. prawns; fish prey consumed are almost exclusively Op. aporos. Ophieleotris aporos is omnivorous and feeds mainly on aquatic insects. Condition, fat deposition, feeding and reproduction in Op. aporos were related to the flood cycle with increases in the flood season; in Ox. heterodon variations in these factors related more to the availability of Op. aporos, seasonal changes being less marked. Ophieleotris aporos matures at a small size, has large gonads, small eggs and is highly fecund; Ox. heterodon exhibited opposite trends in these life-history characteristics by comparison. Reproductive styles adopted by the two species are interpreted in relation to the different predation pressures they experience. Oxyeleotris heterodon may not be adapted to high adult mortality and the fisheries implications of this are discussed.

Interspecific differences in blood hemoglobin concentration ([Hb]) occur among air breathing fishes. However, the effect on [Hb] of factors such as air-breathing organ structure and blood circulation pattern, air breathing behavior, as well as season and environmental conditions have not been fully examined. [Hb] in seven Panamanian species of air-breathing freshwater teleost fishes were compared and were monitored for most species during the wet and dry seasons and in hypoxic laboratory conditions. Fishes studied were Ancistrus, Hypostomus, and Loricaria (Family Loricariidae); Hoplosternum (Callichthyidae); Synbranchus (Synbranchidae); Piabucina (Lebiasinidae); and Dormitator (Eleotridae). [Hb] in these species ranged from 4.8 to 14.6 g 100 ml−1 (g%). Ancistrus, Hypostomus, and Dormitator significantly increased [Hb] during the dry season and, with Piabucina, also increased [Hb] when acclimated to hypoxia in the laboratory. An increase in [Hb] during the dry season may precondition facultative air breathers for habitat hypoxia (and the need to respire aerially) in the event this occurs. Intraspecific differences in both [Hb] and in red cell Hb-phosphate ratio, an index of oxygen affinity, were found in populations of Hypostomus and relate directly to differences in habitat oxygen level. In all species tested alterations in blood hematocrit (Hmct) and mean corpuscular hemoglobin concentration (MCHC) occurred only with correspondingly large net changes in blood [Hb]. The [Hb] of Loricaria, Synbranchus, and Hoplosternum was not affected by season or hypoxia and this may be due to behavioral and physiological adaptations that reduce the transbranchial loss of aerially-obtained oxygen or to a ventilatory mode that precludes this possibility.

Four species of Australian Eleotridae from hypoxic habitats were examined in the laboratory to study buoyancy control in hypoxic water (