Total fish biomass per unit area was positively correlated with total phosphorus, total nitrogen, chlorophyll a, and inversely correlated with Secchi disk transparency in 65 Florida (U.S.A.) lakes selected to range from oligotrophic to hypereutrophic. Species numbers were positively related to lake surface area but not trophic state. There were some shifts in species composition with changes in trophic state, though only a few species showed significant changes in their standing crops. In particular the recreationally important centrarchids did not show important changes with trophic state, and there were no critical points on the trophic spectrum where there were dramatic changes in fish abundance or standing crops. The facts that Florida lakes do not have deep, cold hypolimnia, do not have salmonid species, and have no ice in the winter are among the possible reasons that the more eutrophic Florida lakes do not show the same changes in fish populations often described for northern lakes.Résumé : Il y avait une corrélation positive entre la biomasse totale de poissons par unité de surface et le phosphore total, l'azote total et la chlorophylle a, et une corrélation inverse entre cette biomasse et la transparence mesurée à l'aide du disque de Secchi, dans 65 lacs de Floride (É.-U.) allant d'oligotrophes à hypereutrophes. Les nombres d'espèces étaient liés positivement à la superficie des lacs, mais non à leur état trophique. La composition des espèces variait quelque peu avec l'état trophique, mais seules quelques espèces accusaient des différences importantes dans le nombre d'individus. Plus particulièrement, les centrarchidés importants sur le plan récréatif n'accusaient pas d'importants changements avec l'état trophique, et le spectre trophique ne présentait aucun point critique correspondant à des changements dramatiques de l'abondance ou du nombre d'individus. L'absence d'un hypolimnion froid et profond, l'absence de salmonidés et l'absence de glace en hiver comptent parmi les raisons pouvant expliquer pourquoi les lacs de Floride plus eutrophes n'accusent pas les mêmes variations de population de poissons dont on a souvent fait état dans les lacs nordiques.[Traduit par la Rédaction]

A five-year study examined the responses of submerged aquatic vegetation (SAV), emergent vegetation, and largemouth bass (Micropterus salmoides) to variations in water level in a large lake in Florida, USA. SAV was assessed using a combined transect survey/spatial mapping approach, emergent vegetation was quantified with aerial photography and GIS, and bass were surveyed by electro-shocking. During the period leading up to this study (1995)(1996)(1997)(1998)(1999), water levels were high in the lake, and the SAV was reduced in spatial extent and biomass, compared to its condition in the early 1990s. Spatial extent of emergent vegetation also was low, and largemouth bass surveys indicated low densities and failure to recruit young fish into the population. This was attributed to the lack of critical vegetative habitat. In spring 2000, the lake was lowered by discharging water from major outlets, and this was followed by a regional drought. Water levels dropped by 1 m, and there was widespread development of Chara lawns in shoreline areas, with coincident increases in water clarity. There was some germination of vascular SAV, but Chara was the extreme dominant, such that structural complexity remained low. There was no substantive improvement in bass recruitment. During 2001, water levels declined further, and emergent plants germinated in exposed areas of the lake bottom. SAV was restricted to sites farther offshore, and continued to be dominated by Chara. There again was no bass response. In 2002, conditions changed when water levels increased to a moderate depth, flooding shoreline habitat to $0.5 m. Vascular SAV increased in biomass and spatial extent, such that the community developed a high structural complexity. At the same time, emergent aquatic plants developed dense stands along the western shoreline. Largemouth bass had a strong recruitment of young fish for the first time in 5 years. Recruitment continued to be successful in 2003, when spatial extent of SAV was somewhat reduced by higher water but total biomass and diversity remained high. These results demonstrate an important effect of inter-annual variation in water depth on the population dynamics of aquatic plants and fish in a subtropical lake.

Population dynamics of black crappies Pomoxis nigromaculatus in Lake Okeechobee, Florida, were monitored from 1981 to 1987 to document changes that occurred following suspension of commercial harvest for this species in 1981. Black crappie abundance increased substantially. Mean annual catch per unit effort (CPUE) of otter trawls increased from 1.9 fish/min in 1981 to 22.4 fish/min in 1987. In addition, the estimated winter‐spring angling harvest of black crappies on the north end of the lake increased from 34,000 fish in 1980‐1981 to over 700,000 fish in 1986‐1987. Increased abundance of black crappies resulted from a reduction in annual mortality from 65 to 39% due to suspension of commercial harvest and recruitment of strong yearclasses spawned in 1980, 1983, 1984, and 1985. Multiple linear regression revealed that year‐class strength of black crappies was positively related to water levels during the spring spawning season and to CPUE of January trawls for individuals at least 178 mm in length (total‐length basis); water level was the more important of the two variables. As black crappie density increased from 1981 to 1987, mean lengths in January of age‐2 and age‐3 fish declined by 24 and 38 mm, respectively. Nonetheless, growth did not cease after age 3, and preferred lengths (≥ 250 mm) were attained by age‐4 and older fish. Because black crappie density in 1987 was at its highest level since population monitoring began, commercial harvest could probably be reinstated with little anticipated effect on angler catch rates. However, low to intermediate harvest quotas must be set to avoid the boomor‐bust dynamics created by the original commercial harvest program. Increased mortality due to commercial harvest would also be expected to increase the sizes of black crappie available to anglers.