Commercial fishing results in the incidental capture, injury, and mortality of marine mammals. Often, this occurs at levels constituting an existential threat to individual populations or species. One solution to this global problem is gear modification, which is generally the preferred strategy of the fishing industry. Gear modifications can sometimes achieve bycatch reduction, but commonly suffer from several important limitations: (a) they are often narrowly focused on individual species or populations, have limited applicability to others, and often carry risks to other marine species and ecosystems; (b) they may be implemented in the absence of adequate scientific assessment of their efficacy; (c) testing new gear can actually subject threatened species to increased bycatch risk, a problem compounded by the extreme difficulty of obtaining adequate sample sizes; and (d) a disproportionate focus of this work occurs in developed countries, even though most of the world’s fishing sector (98%) is located in developing countries, as are many of the marine mammals threatened by bycatch. Here, I present global reviews of techniques for reducing marine mammal bycatch in gillnets, and for large whales in multiple gear types. Next, I present results from a field study in a small-scale non-industrial gillnet fishery, in which the catch and bycatch are compared between traditional gillnets and an alternative gear (pots). The most commercially valuable fish species were caught in both, however with differences in size class selectivity. Switching gears may represent a more promising approach than making incremental modifications to existing ones. Lastly, I employ a new computer model that simulates conflicts between the endangered North Atlantic right whale (Eubalaena glacialis) and fishing lines. The model examined their tensile strength under different entanglement scenarios, as an aid for evaluating the potential of using ropes with reduced breaking strength that are practical for fishing but allow large baleen whales to break free if entangled. The results support the use of these “whale-release” ropes to reduce entanglement risk within a large subsector of the US east coast lobster fishery. These studies highlight the benefits of applying new tools and strategies for reducing marine mammal bycatch.

Sustainable and equitable fisheries are central for addressing the challenges of the UN Sustainable Development Goal 14: Life Below Water. International trade, once presented by Walrasian economists as a panacea for fisheries development, has not markedly decreased poverty and has been related to the overexploitation of marine species. In this light the consequences of a continued expansion of seafood trade are highly uncertain and problematic. Two competing theoretical hypotheses predict either overexploitation or recovery of marine species when connected to international trade, respectively. The empirical literature finds trade relationships and connections of local fisheries to a large-volume market critical factors for social-ecological outcomes. Here, I combine these insights to show that multi-level links, between fishers & different markets (market manuscript) and marine species & trade relationships (squid manuscript), are critical to explain diverging social-ecological outcomes. In the market manuscript we model the transition from local, to multi-level (both local and global), to global markets in a two species fishery. We find this transition is non-linear, leading to fluctuations in species abundance as a result of abrupt switches between target species. Critical fluctuations of species abundance driven by new market connections are a result of large shifts in prices for one species and high asymmetries in expected income between the two species. The squid manuscript provides empirical and modeling evidence that cyclical changes in the ocean can drive social-ecological systems outcomes through changing interactions at multiple levels. The interactions between squid population and fishers and squid distribution and trading structures determines benefit distributions in the fishery. The lack of consideration of multi-level interactions related to trade in models for fisheries management is likely associated with a lack of processes for integrating the empirical and theoretical insights of two disciplines at the core of fisheries science. Social-ecological system scholars study more often empirical and fishery economics the theoretical aspects of interactions between trade and fisheries. One process suggested in this thesis to bridge insights from both disciplines in fishery models is the careful study of the important interactions in the empirical case. Comparison of these interactions with observed empirical interactions in other systems informs the model conceptualization that is then embedded in a theoretical framework. This leads to the development of models of intermediate complexity  that integrate insights on regular structures and patterns observed in real social-ecological systems. The squid manuscript exemplifies this integration. We integrate observed multi-level links in a standard fishery model between the squid population fishers and traders, and thus better represent the empirical system.  A continuous dialogue between empirics and theorycan help build models of intermediate complexity. To capture the complex elements of these social-ecological systems, in this young field of study, next to a continuous dialogue priority observed empirical dynamics can help question theoretical assumptions. This study seeks to contribute to the development of fisheries management models more suitable to face contemporary challenges of fisheries management by focusing on how multi-level interactions between fisheries and trade shape sustainable and equitable outcomes.