Cetacean-habitat modeling, although still in the early stages of development, represents a potentially powerful tool for predicting cetacean distributions and understanding the ecological processes determining these distributions. Marine ecosystems vary temporally on diel to decadal scales and spatially on scales from several meters to 1000s of kilometers. Many cetacean species are wideranging and respond to this variability by changes in distribution patterns. Cetacean-habitat models have already been used to incorporate this variability into management applications, including improvement of abundance estimates, development of marine protected areas, and understanding cetacean-fisheries interactions. We present a review of the development of cetacean-habitat models, organized according to the primary steps involved in the modeling process. Topics covered include purposes for which cetacean-habitat models are developed, scale issues in marine ecosystems, cetacean and habitat data collection, descriptive and statistical modeling techniques, model selection, and model evaluation. To date, descriptive statistical techniques have been used to explore cetacean-habitat relationships for selected species in specific areas; the numbers of species and geographic areas examined using computationally intensive statistic modeling techniques are considerably less, and the development of models to test specific hypotheses about the ecological processes determining cetacean distributions has just begun. Future directions in cetacean-habitat modeling span a wide range of possibilities, from development of basic modeling techniques to addressing important ecological questions.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. . Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. The role of omnivory in structuring communities is potentially great in lowland neotropical streams that are characterized by an abundance of macroconsumers that consume both insects and algae. Here, we separate effects of natural densities of diurnal fishes and nocturnal shrimps in structuring the benthic community of a stream draining Costa Rica's Atlantic slope. We experimentally manipulated the spatial and temporal access of fishes and shrimps to benthic resources, in situ, using electric "fences" powered by solar-powered fence chargers.Both fishes and shrimps significantly reduced inorganic sediment mass, organic ashfree dry mass (AFDM), densities of larval Chironomidae, and total insects: their combined effects were greater than effects of either group alone, and there was no significant interaction. Fishes shifted algal community composition from diatoms to green and blue-green algae and benthic insect communities towards chironomids, while shrimps had no significant effect on community composition. Effects of fishes were generally greater than those of shrimps, and this is due, in part, to higher natural densities and foraging pressures of fishes. Furthermore, shrimps foraged for significantly longer periods of time in the treatment where fishes were excluded than in the combined fish and shrimp access treatment, suggesting that diurnally feeding fishes are strong "interactors," mediating resource availability to nocturnally feeding shrimps. Natural erosion and sediment-mediated effects of macroconsumers (both direct and indirect) also affected algal communities: a manual sediment removal experiment resulted in significant reductions of diatom biovolume and increases in the filament length of green and blue-green algae. Our results show the importance of:(1) assessing macroconsumer effects in a relatively natural depositional environment subject to background erosion and sloughing (i.e., in this case by using electric enclosures); (2) evaluating effects of natural densities of both diurnal and nocturnal macroconsumers through time in the context of these abiotic effects; and (3) distinguishing between the response of different types of algal resources (e.g., diatoms vs. green and blue-green algae), which are differentially affected by sedimentation and erosion. Cage experiments, short-term observations, or one-time sampling of undifferentiated "algae" may artificially overestimate trophic effects and underestimate abiotic effects. We found no evidence of a trophic cascade. Our findings are in agreement with the theoretical prediction that large-sized omn...

Habitat prediction models were developed for 13 cetacean species of the mid‐western North Atlantic Ocean: beaked whale, fin whale, humpback whale, minke whale, pilot whale, sperm whale, bottlenose dolphin, common dolphin, Risso's dolphin, spotted dolphin, whitesided dolphin, and harbor porpoise. Using the multiple logistic regression, sightings of cetaceans during the 1990–1996 summer (June‐September) surveys were modeled with oceanographic (sea surface temperature, monthly probability of front occurrence) and topographic (depth, slope) variables for the same period. Predicted habitat maps for June and August were created for each species using a Geographical Information System. The predicted habitat locations matched with current and historic cetacean sighting locations. The model also predicted habitat shifts for some species associated with oceanographic changes. The correct classification rate of the prediction models with 1997–1998 summer survey data ranged from 44% to 70%, of which most of the misclassifications were caused by false positives (i.e., absence of sightings at locations where the models predicted).

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology. Abstract.We examined how biotic factors (natural assemblages of omnivorous fishes) control the community response of primary producers to physical disturbance (frequent high discharge events) in a lowland tropical stream. As a secondary objective, we assessed effects of fishes on benthic insects. We used electric "fences" to exclude access of fishes to benthic communities in a riffle of a fourth-order stream draining the foothills on Costa Rica's Caribbean slope. Benthic algae and aquatic insects accessible to fishes were subject to intense omnivory by a diverse assemblage of fishes. Over a 7-wk period in the rainy season, three large discharge events occurred in which water levels rose up to 3 m above base flow, discharge increased by up to 160-fold, and suspended gravel and small cobbles caused major scouring. Fishes significantly affected algal succession, community composition, and response to high discharge events. Algal assemblages that developed in the presence of fishes were dominated by filamentous blue-green algae (Lyngbya sp.), whereas diatom assemblages prevailed where fishes were excluded. Fishes significantly reduced numbers of larval Chironomidae (Diptera) and total insects. Algal biovolume was higher and more variable in the absence of fishes than in their presence: significant (P < 0.05) reductions in algal biovolume occurred in the diatomdominated fish exclusion treatment in response to high discharge. In contrast, in the presence of fishes, algal biovolume increased steadily through time and was not significantly reduced by storms. The third and most severe storm event (39-40 d) resulted in decreases in total algal biovolume, diatom biovolume, algal taxon diversity, and richness in the fish exclusion treatment. However, these parameters increased (despite the storm) in the presence of fishes. Our results show that fishes play a key role in maintaining the stability of benthic algal assemblages and their resistance to storm events. Moreover, results suggest that in the absence of omnivorous fishes, high discharge events would play a major role in structuring benthic algal assemblages, resulting in extreme fluctuations in algal biomass.

The average sizes of Pacific salmon have declined in some areas in the Northeast Pacific over the past few decades, but the extent and geographic distribution of these declines in Alaska is uncertain. Here, we used regression analyses to quantify decadal trends in length and age at maturity in ten datasets from commercial harvests, weirs, and spawner abundance surveys of Chinook salmon Oncorhynchus tshawytscha throughout Alaska. We found that on average these fish have become smaller over the past 30 years (~6 generations), because of a decline in the predominant age at maturity and because of a decrease in age-specific length. The proportion of older and larger 4-ocean age fish in the population declined significantly (P < 0.05) in all stocks examined by return year or brood year. Our analyses also indicated that the age-specific lengths of 4-ocean fish (9 of 10 stocks) and of 3-ocean fish (5 of 10 stocks) have declined significantly (P < 0.05). Size-selective harvest may be driving earlier maturation and declines in size, but the evidence is not conclusive, and additional factors, such as ocean conditions or competitive interactions with other species of salmon, may also be responsible. Regardless of the cause, these wide-spread phenotypic shifts influence fecundity and population abundance, and ultimately may put populations and associated fisheries at risk of decline.