Worldwide, finfish fisheries receive increasing assessment and regulation, slowly leading to more sustainable exploitation and rebuilding. In their wake, invertebrate fisheries are rapidly expanding with little scientific scrutiny despite increasing socio-economic importance. This thesis provides the first global analysis of the trends, drivers, and population and ecosystem consequences of invertebrate fisheries, in general, and sea cucumber fisheries, in particular, based on a global catch database in combination with taxa-specific reviews. Further, I developed new methods to quantify trends over space and time in resource status and fishery development. Since 1950, global invertebrate catches increased six-fold with 1.5 times more countries fishing and double the taxa reported. By 2004, 31% of fisheries were over-exploited, collapsed, or closed. New fisheries developed increasingly rapidly, with a decrease of six years (± three years) in time from start to peak from 1960 to 1990. Moreover, 71% of invertebrate taxa (53% of catches) are harvested with habitat-destructive gear, and many provide important ecosystem functions including habitat, filtration, and grazing. For sea cucumber fisheries, global catch and value has increased strongly over the past two to three decades, closely linked to increasing prices and demand on Asian markets. However, the catch of individual fisheries followed a boom-and-bust pattern, declining nearly as quickly as it expanded, and expanding approximately five times as quickly in 1990 compared to 1960. Also, new fisheries expanded increasingly far from their driving market in Asia, and encompassed a global fishery by the 1990s. One-third of sea cucumber fisheries experienced declines in average body size fished; half showed serial exploitation over space by moving further away from the coast; three-quarters showed serial exploitation from high- to low-value species; and two-thirds experienced population declines due to overexploitation with local extirpation in some cases. One-third of all sea cucumber fisheries remain unregulated. These findings suggest that the basis of marine food webs is increasingly exploited with limited stock and ecosystem-impact assessments, and a new management focus is needed to avoid negative consequences for ocean ecosystems and human well-being.

In this thesis I assessed the causes of long-term changes in two large, commercially important decapod crustacean populations, American lobster (Homarus americanus) and snow crab (Chionoecetes opilio), in the northwest (NW) Atlantic Ocean. By combining available time-series data, including commercial landings, research surveys, and local ecological knowledge (LEK), I explored the causes of an observed ecosystem shift in the NW Atlantic (~1950–2009) which entailed a region-wide decline of groundfish and an increase in benthic invertebrates, including these decapods. Three hypotheses were examined to explain the increase in decapod abundance: (1) the predation hypothesis, whereby a decrease in predatory groundfish led to an increase in their decapod prey (top-down effects); (2) the climate hypothesis, whereby changes in temperature or other climatic variables helped to increase decapod numbers (bottom-up effects); and (3) the anthropogenic hypothesis, whereby changes in fishing pressure drove decapod population dynamics. I explored these hypotheses separately for lobster and snow crab, which may experience different ecological and commercial pressures. First, I investigated the interactions between predatory groundfish and lobster in the inshore region of southwest Nova Scotia. Long-term fisheries-independent abundance indices for lobsters and their predators are available for Gulf of Maine (GOM) waters in the USA, but not in Canada. To address research gaps I designed and executed a survey to collect the LEK of lobster fishermen fishing in the Canadian GOM. Forty-two fishermen were interviewed. Corresponding survey results from the USA were compared to the LEK results. Both sources provided evidence for a top-down effect (predation release), contributing to observed increases in GOM lobster abundance and landings. Second, I explored relationships between lobster abundance and landings in the NW Atlantic as they may relate to temporal changes in predators, temperature, climate (North Atlantic Oscillation Index, NAOI), and fishing. Available landings data and fisheries-independent abundance estimates were collated to investigate trends in lobster abundance and catch. Links between lobster, groundfish, temperature and climate indices were explored using mixed effects models. Results offered partial support for the predation hypothesis, namely in the waters off Newfoundland, Nova Scotia, and southern New England as well as broad support for a climate effect on early life stages. This effect appeared related to a region-wide climate signal, the NAOI, but was independent of changes in water temperature. Fishing effort appeared to be following lobster abundance, rather than regulating abundance in a consistent way. Third, variation in snow crab abundance was examined through meta-analysis of time-series data of cod and crab abundance and temperature. Temperature had opposing effects on the two species: snow crab abundance was negatively correlated with temperature whereas cod and temperature were positively related. Controlling for the effect of temperature, the analysis revealed significant negative interactions between snow crab and cod abundance, with cod leading snow crab up to a five-year lag. Results indicate that snow crab is largely influenced by temperature during early post-settlement years and becomes increasingly regulated by top-down mechanisms as they approach fishery recruitment. Overall, I conclude that both climate and predation can act as population controls on large decapod populations, but these variables affect decapods at different life stages.