Influence of puffer predation on vertical distribution of mangrove littorinids in the Gulf of Nicoya, Costa Rica

The eastern Pacific puffer fish Sphoeroides rosenblatti (family Tetraodontidae) preys on the mangrove periwinkles Littorariafasciata and L. varia up to 13 cm above the water surface. Puffers capture snails by jumping partially out of the water and removing them by mouth from Rhizophora spp. prop roots. Field predation trials suggested puffer predation is intense, averaging 40.6% h⁻¹ on L. fasciata tethered at the water surface. L.fasciata <∼8 mm in shell length and L. varia <∼12 mm remained near or below the water surface and were easily crushed by puffers; gut content analyses showed that these snails were commonly eaten by puffers. L.fasciata >∼8 mm and L. varia∼12–16 mm were also crushed by puffers, but were generally located above their reach; these snails were much less abundant in puffer guts. L.varia >∼16 mm were abundant and some were submerged at high tide; however, none were found in puffer guts presumably because puffers encounter difficulty crushing these snails. S.rosenblatti is an important predator of L. fasciata and L. varia and appears to affect their vertical distributions. Received: 21 April 1998 / Accepted: 10 August 1998     

TRENDS IN MANATEE (TRICHECHUS MANATUS LATIROSTRIS) COUNTS AND HABITAT USE IN TAMPA BAY, 1987–1994: IMPLICATIONS FOR CONSERVATION

Aerial surveys ( n = 88) were used to document locations and count sightings of manatees ( Trichechus manatus latirostris ) in the inshore waters of Tampa Bay, Florida, between November 1987 and May 1994. We made 5,358 sightings of manatees in 1,958 groups. Calves represented 8% of the manatees sighted. Counts were significantly higher in winter ( = 79, n = 29 flights) than in non-winter ( = 46, n = 47) months. Counts of manatees in winter increased significantly during the study, but warm-season counts did not. Regression models demonstrated a relationship between counts and environmental factors. Year-round counts were related to air temperatures and seasons, with highest counts in winter. However, in the winter season, counts were significantly correlated only with wind speed, not air temperature. Yearround counts were predicted to be curvilinear with highest counts at 15°C average air temperature. Areas used differed with season: in cold weather, 76% of all sightings occurred in zones with warm-water sources. High-use areas were identified for summer months. Spatial filter analysis was used to compare manatee density in high-use areas between two two-year time periods. The data indicate that (1) manatee use of Tampa Bay was high and increasing in winter, (2) there are particular zones of the bay where conservation of manatees and habitat should be a priority, and (3) sufficient information has been collected for management agencies to develop and implement manatee protection plans.     

Ontogenesis of osmotic regulation in the striped mullet, Mugil cephalus L.

The osmoregulatory capabilities of juvenile striped mullet, Mugil cephalus L ., of three size-groups (20–29, 30–39 and 40–9 mm s.l.) were compared in a series of six salinities ranging from fresh water to full sea water. The two smaller size-groups were able to tolerate instantaneous transfer from the brackish water in which captured, to all salinities but fresh water, while the 40–69 mm group were tolerant of instantaneous transfer to all experimental salinities. At high environmental salinities, the osmotic regulatory capabilities improved with growth in size to an apparently definitive condition in fish of 40–69 mm s.l. The smallest of these individuals were estimated to be about 7.5 months old. The osmotic regulatory capability in waters of low salinity had reached a definitive state with the development of tolerance to such salinities. The ontogenetic pattern for Mugil cephalus is virtually identical to that of the Atlantic salmon, Salmo salar .