Reduced growth in wild juvenile sockeye salmon Oncorhynchus nerka infected with sea lice.

Daily growth rings were examined in the otoliths of wild juvenile sockeye salmon Oncorhynchus nerka to determine whether infection by ectoparasitic sea lice Caligus clemensi and Lepeophtheirus salmonis was associated with reduced host body growth, an important determinant of survival. Over 98% of the sea lice proved to be C. clemensi and the fish that were highly infected grew more slowly than uninfected individuals. Larger fish also grew faster than smaller fish. Finally, there was evidence of an interaction between body size and infection status, indicating the potential for parasite-mediated growth divergence.

Does predation risk influence habitat use by northern redbelly dace Phoxinus eos at different spatial scales?

This study investigated the relationship between spatial variations in predation risk and abundance of northern redbelly dace Phoxinus eos at both macroscale (littoral v. pelagic zones) and microscale (structured v. open water habitats in the littoral zone) of Canadian Shield lakes. Minnow traps were placed in both structured and open water habitats in the littoral zone of 13 Canadian Shield lakes, and estimates of the relative predation risk of P. eos in both the pelagic and the littoral zones were obtained from tethering experiments. Results showed that (1) the mean abundance of P. eos in the littoral zone was positively correlated with the relative predation risk in the pelagic zone, (2) P. eos preferentially used structured over open water habitats in the littoral zone and (3) this preference was not related to the relative predation risk in the littoral zone but decreased as the relative predation risk increased in the pelagic zone. At the lake level, these results support the hypothesis that P. eos enter the littoral zone to avoid pelagic piscivores. At the littoral zone level, the results do not necessarily contradict the widely accepted view that P. eos preferentially use structured over open habitats to reduce their predation risk, but suggest that flexibility in antipredator tactics (e.g. shelter use v. shoaling) could explain the spatial distribution of P. eos between structured and open water habitats.

Sea lice escape predation on their host.

Parasites seldom have predators but often fall victim to those of their hosts. How parasites respond to host predation can have important consequences for both hosts and parasites, though empirical investigations are rare. The exposure of wild juvenile salmon to sea lice (Lepeophtheirus salmonis) from salmon farms allowed us to study a novel ecological interaction: the response of sea lice to predation on their juvenile pink and chum salmon hosts by two salmonid predators-coho smolts and cut-throat trout. In approximately 70% of trials in which a predator consumed a parasitized prey, lice escaped predation by swimming or moving directly onto the predator. This trophic transmission is strongly male biased, probably because behaviour and morphology constrain female movement and transmission. These findings highlight the potential for sea lice to be transmitted up marine food webs in areas of intensive salmon aquaculture, with implications for louse population dynamics and predatory salmonid health.

The development of a new geographic coding system for the Continuous Work History Sample.

This article describes the statistical development of the geographic coding system used to identify worker location for the Continuous Work History Sample. The new system--which is planned for implementation for data year 1993--will provide more accurate geographic distributions of workers within a residence concept than the old system could provide within an employer location concept. The article also presents the results of a pilot study that tested the operational aspects of the new system. The results provide some preliminary estimates of the effect of the revised codes on the geographic distribution of workers.

The development and use of industry data by the Social Security Administration.

Over the past few years the Social Security Bulletin has published a series of technical articles that describe various Social Security Administration (SSA) data files. This article provides an overview of SSA's industry-related data files and statistical systems from both a current and a historical perspective. The author begins by explaining how SSA first collected business data from employers (starting in 1937) as a by-product of the requirement that employers report employee wages for benefit computation purposes. She describes the administrative methods by which the data are collected, SSA's coordination of its activities with other agencies, the data collection forms used, the scheme by which the data are coded, and the employer files into which the data are classified. In her closing, the author provides examples of the various uses of the industry data and the ways that these data relate to SSA's statistical program needs and to those of other agencies as well.