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Climate change, fishing, and aquaculture have affected and will continue to influence Canadian marine biodiversity, albeit at different spatial scales. The Arctic is notably affected by reduced quality and quantity of sea ice caused by global warming, and by concomitant and forecasted changes in ocean productivity, species ecology, and human activity. The Atlantic has been especially impacted by severe overfishing and human-induced alterations to food webs. Climate change, fishing, and aquaculture have all affected, to varying degrees, biodiversity on Canada’s Pacific coast. Past and projected trends in key biodiversity stressors reveal marked change. Oceanographic trends include increasing surface water temperatures, reduced salinity, increased acidity, and, in some areas, reduced oxygen. Reductions in Canada’s fishery catches (those in 2009 were half those of the late 1980s), followed by reductions in fishing pressure, are associated with dramatic changes in the species composition of commercial catches in the Atlantic (formerly groundfish, now predominantly invertebrates and pelagic fish) and the Pacific (formerly salmon, now predominantly groundfish). Aquaculture, dominated by the farming of Atlantic salmon, grew rapidly from the early 1980s until 2002 and has since stabilized. Climate change is forecast to affect marine biodiversity by shifting species distributions, changing species community composition, decoupling the timing of species’ resource requirements and resource availability, and reducing habitat quality. Harvest-related reductions in fish abundance, many by 80% or more, coupled with fishing-induced changes to food webs, are impairing the capacity of species to recover or even persist. Open-sea aquaculture net pens affect biodiversity by (i) habitat alteration resulting from organic wastes, chemical inputs, and use of nonnative species; (ii) exchange of pathogens between farmed and wild species; and (iii) interbreeding between wild fish and farmed escapees. Physical and biological changes in the oceans, along with direct anthropogenic impacts, are modifying Canadian marine biodiversity with implications for food security and the social and economic well-being of coastal communities. To assess the consequences of changes in biodiversity for Canada’s oceans and society, it is necessary to understand the current state of marine biodiversity and how it might be affected by projected changes in climate and human uses.

The recovery of depleted species depends on their population dynamics at low abundance. Classical population growth models, applied widely in fisheries science, assume that per capita offspring production increases as abundance declines (compensation). However, slow or absent recovery by many depleted fishes might reflect unexpectedly weak compensation or the presence of Allee effects (depensation). Using meta-analytical techniques to describe reproductive dynamics, we find considerable variability among 207 exploited marine fish stocks (104 species) in how standardized per capita population growth changes with abundance. Although many species exhibit strong compensatory dynamics (negative density dependence), others show much weaker compensation than expected, and some exhibit evidence of an Allee effect, such as Atlantic cod ( Gadus morhua ) and Alaskan walleye pollock ( Theragra chalcogramma ). As data at low levels of abundance become increasingly available, it appears that compensation, while strong in some species, is comparatively weak or nonexistent in others, thus providing an explanation for why the recovery of some depleted stocks, despite reductions in exploitation, has been considerably less than what classic models of population growth would otherwise suggest.

Population mean egg size of salmonids increases with the mean size of gravels in which a population spawns. A long-standing hypothesis is that large larvae cannot navigate small-gravel interstices, so mothers must decrease per-offspring investment when spawning gravels are small. We manipulated the size of incubation gravels and egg size of Atlantic salmon (Salmo salar) to test whether size-related entombment of larvae occurs. We find little evidence of size-related entombment, but we find evidence that gravel size does not affect all larval sizes equally. Larger larvae emerge from small gravels before development is complete and with a visible yolk sac, possibly due to oxygen limitation in small gravels. Smaller larvae always complete development in the gravel and emerge without a yolk sac. Although growth and survival may increase with juvenile size following yolk-sac absorption, juveniles with yolk sacs may fare worse when depredation rates are high. The egg size – gravel size correlation may therefore reflect increased postemergence mortality among larger offspring in small-gravel environments. Alternatively, compaction stress in fine gravels coupled with size asymmetries in larval strength may have caused the patterns that we observed, in which case our data may not help explain the egg size – gravel size correlation.

Various mechanisms of sexual selection can occur at both the whole organism and gamete levels. Fertilization success in salmonid fishes is largely determined by behavioural competition within and between “fighter” and “sneaker” male strategies, but is also influenced by interactions among gametes. We investigated the influence of density, fighter male presence, and major histocompatibility (MH) genotype on patterns of fertilization success in sneaker Atlantic salmon ( Salmo salar ). At low density and in the absence of fighter males, monopolization by single sneaker males occurred, suggesting that male–male competition was the main determinant of fertilization success. By contrast, at high density and (or) in the presence of fighter males, several sneakers succeeded in fertilizing eggs. Our study, the first to investigate the role of MH-mediated fertilization for a genetically based alternative male mating strategy, suggests that a larger proportion of eggs were fertilized by sneakers whose MH genotypes were more similar to those of the female than expected by chance. These findings highlight the importance of examining sexual selection under different competitive conditions.