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Sex differences in early development may play an important role in the expression of sexual size dimorphism at the adult stage. To test whether sexual size dimorphism is present in pre-emergent chinook salmon (Oncorhynchus tshawytscha), alevins were reared at two temperatures (10 °C and 15 °C) and sexed using the OtY1 marker on the Y-chromosome. Linear mixed models were used to test for sex differences in alevin size within families while controlling for the random effects of sire and dam nested within sire. Males and females did not differ in weight at 10 °C but males were heavier than females at 15 °C. Sex accounted for 2% of the within-family variance in weight. In addition, at 15°C, the relationship between weight and sex was greater in families with larger eggs. Whereas male-biased sexual size dimorphism was present at the juvenile stage, female-biased sexual size dimorphism was present at sexual maturity. Males were also younger than females at sexual maturity. A head start on growth by males may underlie their earlier maturation at a smaller size, thus leading to female-biased SSD at the adult stage.

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Microcystin toxins from harmful algal blooms (HABs) can accumulate and persist in fish, raising dual concerns about human health risks from consumption and the potential for detrimental impacts on fish populations. However, there are fundamental unknowns about the relationship between HABs and fish populations driven by a lack of field information on toxin accumulation and retention over space and time. We conducted a field study to assess human health risks from consuming fish caught across all life stages of a HAB and to determine the pervasiveness of potentially harmful levels of microcystins on fish populations. We collected 190 fish in 2015 and 2017 from Lake Erie, a large freshwater ecosystem that is highly productive for fisheries and is an epicenter of HABs and microcystin toxicity events. Muscles and livers were analyzed for total microcystins, which was used to conduct a human health risk assessment for comparison against fish consumption advisory benchmarks available for Lake Erie. We find low human health risk from muscle consumption following the World Health Organization's safety thresholds. However, all fish across capture dates had microcystins in their livers at levels shown to cause adverse effects, suggesting a pervasive and underappreciated toxic stressor. These data demonstrate that microcystins are retained in fish livers well beyond the cessation of HABs and calls for additional research to better understand the effects of sublethal toxic exposures for fish population dynamics, conservation, and related ecosystem services.

Large‐scale stocking programs have been used to introduce and maintain Chinook Salmon Oncorhynchus tshawytscha fisheries in the Laurentian Great Lakes since the late 1960s, with discoveries of naturalized populations in tributaries soon after the initial introduction. Characterizing ecological differences between hatchery and naturalized Chinook Salmon is becoming increasingly important for sustainable management of this species. We hypothesized that nonheritable traits (i.e., homing) and artificial selection might result in different habitat use and arrival timing between rearing origins occupying the same river, with the potential for reproductive isolation and divergence of naturalized and hatchery strains. In 2010 and 2011, using visual observations and PIT tags, we tracked individual hatchery and naturalized Chinook Salmon that spawned in the Sydenham River (Owen Sound, Ontario), one of the first tributaries in Lake Huron to be colonized by hatchery strays and establish a naturalized population. We estimated that 52–58% of the 3,200–3,300 fish returning to spawn in 3.5 km of accessible spawning habitat were hatchery fish. In females, the extent of prespawning movement increased with arrival date, decreased with fish length, and was lower for hatchery fish. Hatchery females also displayed a higher probability of direct homing to the stocking site than naturalized females. Prespawning movements by males were extensive but did not depend on rearing origin, arrival timing, or body size. We found no evidence of reproductive isolation, however. Spatial distributions of spawning locations were independent of rearing origin for both sexes, and arrival timing in the river was similar between rearing origins for both sexes. The lack of reproductive isolation is likely attributable to the small area of suitable habitat in this river. Our study represents the first comprehensive comparison of spawning habitat use and timing by hatchery and naturalized Chinook Salmon in the Great Lakes. Received October 26, 2016; accepted February 22, 2017 Published online April 24, 2017