Restoration of Zooplankton Communities in Industrially Damaged Lakes: Influences of Residual Metal Contamination and the Recovery of Fish Communities

The Sudbury, Ontario, Canada area offers a unique opportunity to develop our understanding of biotic and abiotic lake recovery processes in industrially damaged natural systems. In recent decades, lakes in the Sudbury area have shown improvements in water quality due to decreases in sulfur (S) and metal emissions from area smelters, and reduced acid deposition from more distant sources. However, biological recovery is lagging and mechanisms controlling the lag are not yet clear. Our study examines the roles of two factors, residual metal contamination and altered fish predation, on zooplankton community recovery. Data collected over three decades on six study lakes were analyzed using redundancy analysis (RDA) and partial RDA's to assess historical and present relationships of water chemistry and fish abundance with zooplankton community recovery. Continuing metal toxicity appears to be the primary cause of the absence of some zooplankton species, particularly Daphnia spp. from metal‐contaminated lakes. Conversely, once water quality is suitable and abundant planktivores reestablish, fish planktivory becomes a factor affecting Daphnia spp. establishment. The introduction of piscivores into these lakes may be necessary to facilitate the return of many Daphnia species. Further reductions in metal toxicity will also assist with the complete recovery of zooplankton communities. Studying natural systems over several decades allows us to better understand the intricate steps involved with recovery of industrially damaged lakes, and this knowledge will greatly benefit future restoration efforts in other industrially damaged systems .     

Sex‐specific prey partitioning in breeding piscivorous birds examined via a novel,noninvasive approach

Piscivorous birds frequently display sex‐specific differences in their hunting and feeding behavior, which lead to diverging impacts on prey populations. Cormorants (Phalacrocoracidae), for example, were previously studied to examine dietary differences between the sexes and males were found to consume larger fish in coastal areas during autumn and winter. However, information on prey partitioning during breeding and generally on sex‐specific foraging in inland waters is missing. Here, we assess sex‐specific prey choice of Great Cormorants (Phalacrocorax carbo) during two subsequent breeding seasons in the Central European Alpine foreland, an area characterized by numerous stagnant and flowing waters in close proximity to each other. We developed a unique, noninvasive approach and applied it to regurgitated pellets: molecular cormorant sexing combined with molecular fish identification and fish‐length regression analysis performed on prey hard parts. Altogether, 364 pellets delivered information on both, bird sex, and consumed prey. The sexes differed significantly in their overall prey composition, even though Perca fluviatilis, Rutilus rutilus, and Coregonus spp. represented the main food source for both. Albeit prey composition did not indicate the use of different water bodies by the sexes, male diet was characterized by higher prey diversity within a pellet and the consumption of larger fish. The current findings show that female and male cormorants to some extent target the available prey spectrum at different levels. Finally, the comprehensive and noninvasive approach has great potential for application in studies of other piscivorous bird species.     

Foraging requirements of the endangered long-fingered bat: the influence of micro-habitat structure, water quality and prey type

Myotis capaccinii is one of the most endangered Mediterranean bats. We radio tracked 21 adult individuals to assess foraging range and analyse micro-habitat selection around a nursery roost in central Italy. Habitats were characterized by development of riparian vegetation, distance between banks, degree of water clutter and levels of water pollution (expressed by macro-benthic bioindicators). Diet was also analysed to see whether prey rarity might account for species decline. Myotis capaccinii proved highly mobile: the maximum straight line distance between roost location and the farthest foraging fix recorded on one night was c . 21 km, whereas this variable averaged 7.5 km for the entire study. In general, the species preferred calm waters bordered by well-developed riparian vegetation and large (>5 m) inter-bank distances. Such factors determined the overall quality of foraging sites. Although diet was dominated by pollution-tolerant insects such as Chironomus midges, bats preferred less polluted waters. Our study also confirmed the occurrence of piscivory in this bat. Of the factors potentially responsible for species endangerment, the most likely are general habitat degradation, that is alteration of riparian vegetation and pollution, and cave roost loss. Given the large ecological niche overlap, increasing Myotis daubentonii may be outcompeting M. capaccinii . The most urgent actions to preserve M. capaccinii are extensive restoration of riparian vegetation and strict roost protection. Our study offers indications about where, and how, restoration of riparian vegetation may translate into best conservation results.     

Spatial and seasonal patterns in the feeding habits of juvenile Lates niloticus (L.), in the Mwanza Gulf of Lake Victoria

Flexibility in the feeding habits of juvenile Nile perch (1–30 cm total length) was studied from September 1988 to September 1989 at four sites (depth range: 1–25 m) in the Mwanza Gulf of Lake Victoria. During this period haplochromine cichlids were virtually absent in the area. We looked at the combined effects of predator size, season and habitat. Stomach content analysis showed that with increase in size, the diet of Nile perch shifted from zooplankton and midge larvae, to macro-invertebrates (shrimps and dragonfly nymphs) and fish. At a size of 3–4 cm Nile perch shifted from size-selective predation on the largest cyclopoids to predation on the largest, less abundant, calanoids. Zooplanktivory ended at a size of ca. 5 cm. Although an ontogenetic shift in the diet of juvenile Nile perch was obvious at all sampling stations, the contribution of prey types appeared to be habitat related. With increasing water depth the frequency of occurrence in the diet of most prey types decreased, but that of shrimps increased. At the entrance of the gulf (20–25 m deep) shrimps were the main food source throughout the year. Halfway the gulf (12–16 m), Nile perch showed seasonality in their feeding behaviour. Shrimps were taken there especially during the rainy season (January to May) when their densities at this station were high, whereas cannibalism prevailed during the rest of the year. In an environment with Nile perch and dagaa as alternative prey, shrimps were taken almost exclusively. They could be regarded as a key prey for Nile perch between 5 and 30 cm.     

Fish feeding guilds in shallow rocky and soft bottom areas on the Swedish west coast

Multivariate analyses of the diet relationships within fish assemblages resulted in the recognition of five major feeding guilds in shallow (0-9 m) rocky- and soft-bottom habitats on the Swedish west coast. The two habitats had similar guilds that could be broadly described as piscivores, decapod feeders, copepod feeders and amphipod and mollusc feeders. The last guild on the rocky bottom was more strictly one of amphipod feeders, whereas the last guild on soft bottoms consisted of infauna feeders (mainly polychaetes and bivalves). A comparison of the diet of the entire fish assemblages showed that amphipods and gastropods were more prevalent as prey taxa on rocky bottoms, whereas bivalves and teleosts were more common on soft bottoms. A large proportion (12 out of 27 species) of the fish species investigated was found in both habitats. These fish species generally seemed to rely on the same major prey groups in the two habitats, but also included prey taxa characteristic for the habitat in which they resided. Sea trout Salmo trutta , mackerel Scombrus scombrus , saithe Pollachius virens , whiting Merlangius merlangus and cod Gadus morhua were found to be the major piscivores in both the investigated habitats. Herring Clupea harengus and members of the family gobiidae were the most prevalent prey species for piscivores within the rocky- as well as the soft-bottom fish assemblage. A comparison of feeding modes suggested that the fish assemblage on rocky bottoms predominantly (60% of the food items) relied on food found in vegetation. On soft bottoms, the average distribution of food categories among fish species was 44% benthic, 35% pelagic and 21% vegetation-associated food items.     

Diet of the twaite shad Alosa fallax (Lacépède) (Clupeidae) in the River Tagus Estuary, Portugal

The diet of Alosafallax was studied by means of the frequency of occurrence of prey items. In the Tagus Estuary, shads feed preferentially on fish, with mysids, shrimp, isopods and insects occurring as secondary prey.     

Piscivory and cannibalism in Arctic charr

Piscivory and cannibalism in Arctic charr, Salvelinus alpinus , were studied in three lakes in northern Norway: Guolasjavri, which contains only charr, Takvatn, where Arctic charr coexist with three-spined sticklebacks, Gasterosteus aculeatus and brown trout, Salmo trutta , and Stuorajavri, where whitefish, Coregonus lavarelun dominate a fish community containing six species. The prevalence of piscivory in the Arctic charr populations generally increased with increasing predator size. In all three lakes, many charr larger than 20 cm were piscivorous, but the extent of piscivory and cannibalism varied. The greatest prevalence of cannibalism was found in Guolasjavri, where 27% of charr greater than 20 cm in length had fed upon smaller conspeciflcs. In Takvatn, 5% of charr larger than 20 cm were cannibalistic, and an additional 9% had eaten three-spined sticklebacks. In Stuorajavri, up to 74% of the charr greater than 20 cm had eaten whitefish but cannibalism was not recorded. The possible role of cannibalism in population regulation within Arctic charr populations is considered.     

Prey size selection and cannibalistic behaviour of juvenile barramundi Lates calcarifer

This study assessed the cannibalistic behaviour of juvenile barramundi Lates calcarifer and examined the relationship between prey size selection and energy gain of cannibals. Prey handling time and capture success by cannibals were used to estimate the ratio of energy gain to energy cost in prey selection. Cannibals selected smaller prey despite its capability of ingesting larger prey individuals. In behavioural analysis, prey handling time significantly increased with prey size, but it was not significantly affected by cannibal size. Conversely, capture success significantly decreased with the increase of both prey and cannibal sizes. The profitability indices showed that the smaller prey provides the most energy return for cannibals of all size classes. These results indicate that L. calcarifer cannibals select smaller prey for more profitable return. The behavioural analysis, however, indicates that L. calcarifer cannibals attack prey of all size at a similar rate but ingest smaller prey more often, suggesting that prey size selection is passively orientated rather than at the predator's choice. The increase of prey escape ability and morphological constraint contribute to the reduction of intracohort cannibalism as fish grow larger. This study contributes to the understanding of intracohort cannibalism and development of strategies to reduce fish cannibalistic mortalities.     

Food Preference of Perch (Perca fluviatilis) Stocked in a Previously Fishless,Acid Bog Lake and Development of the Fish Population in Absence of Piscivory

The almost fishless, acid Lake Fuchskuhle was physically partitioned into 4 sections and stocked with young, 10‐ to 13‐cm perch. Their body growth was normal only up to age 4 years. Although the perch spawned successfully, reproduction was negligible because only few offspring survived the first year of life. Food for young perch was plentiful, benthic invertebrates composing the main source. Because the population included only 2 to 3 year classes, the perch could not shift from benthic feeding to piscivory as they grew. As a result, growth rate decreased in older fishes (over 4–5 years old). The benthic food supply alone could not satisfy the energy requirements of the older perch.