Ecological Connectivity in the Atlantic Ocean: Past, Present and Future

Marine life is shaped by interactions among organisms and between the organisms and their habitats. This thesis focuses on understanding the interconnections and how they are influenced by the dynamics and functionality of marine ecosystems across spatial and temporal scales.

Marine connectivity indeed accounts for the intricate network of ecological linkages within marine ecosystems, as these systems do not function in isolation but are interdependent on their biological and physicochemical variables to collectively support their resilience and health. In most marine organisms’ connectivity between species and across habitats is often related to the dispersal of the different individual early life stages, like eggs, larvae and propagules, as well as juvenile and adult stages.

The research conducted in this thesis focuses on the ecological connectivity of different marine organisms, and specifically the dispersal of early life stages in several fish species and a mollusk. This is done by exploring how connectivity influences survival, recruitment, and adaptation of these marine communities and populations. Results could help to understand the variability of the abundance of early life stages, which is especially important for the sustainable management of commercial species. The results could also provide insights into the potential dispersal of non-indigenous species and could potentially inform the establishment of marine protected areas in coastal and open ocean areas.

The thesis is structured into three manuscripts; the first manuscript provides an overview of the primary commercial fish species in the Atlantic Ocean, encompassing 17 species represented by 113 stocks. The study included Southern African anchovy, European anchovy, bigeye tuna, Atlantic cod, American eel, European eel, haddock, Atlantic herring, Argentinian hake, Atlantic mackerel, Gulf menhaden, sardine, pollock, round sardinella, skipjack tuna, European sprat, and yellowfin tuna. The first manuscript highlighted the existence of different reproductive strategies in these fish groups, specially assessing whether the larvae of these species could reach areas of higher, lower or equal productivity compared to their spawning grounds.

Studying the population connectivity in the Atlantic Ocean, comparing the productivity of the initial and final positions (∆PP), we found that from the 113 stocks, an increase in ∆PP (n=31), a decrease (n=64) or values close to zero (n=18) can all produce a viable strategy for fish to survive. From an ecological point of view, this implies that larvae can be transported to regions with higher, lower or similar primary productivity as those experienced in the spawning ground (manuscript I).

In the second manuscript, we focused on two stocks of Argentine shortfin squid (Illex argentinus), that has been less studied although it plays a substantial role in the Brazilian, Uruguayan and Argentine commercial fisheries. We focused on understanding conditions for maximizing recruitment and whether an optimal dispersal strategy can be identified solely based on behavior and transport. Specifically, we tested various scenarios, including different diel vertical migration patterns and fixed depth strategies.

Results demonstrated that differences in diel vertical migration do not have a major role in controlling recruitment of the species, within the multi-year period that has been simulated. On the other hand, spawning time showed marked changes in recruitment success. Specifically, when risks related to year-to-year variability are considered, early spawners have greater advantages in recruitment compared to late spawners (manuscript II).

In the third manuscript, we investigated the effects of changes in the sea currents on eggs and larval transport of European eel (Anguilla anguilla). The early life stages of this species can drift over 4,000 kilometers from the spawning ground in the Sargasso Sea to the recruitment regions in European coastal areas. With climate change projected to alter ocean circulation, we investigated how these changes could impact the larval drift. Additionally, we analyzed the minimum and maximum temperatures experienced by the eggs and larvae to assess potential changes in their thermal environment, as temperature is a critical factor influencing larval development and survival.

Comparing the number of successful particles able to reach 25°W (west Iceland) in the decades of 2000s and 2040s, we found that the 2000s would have 48% more successful particles than the 2040s. We also observed that eggs would experience one-degree warmer temperatures, while larvae would experience the same range of temperature. Overall, the number of particles arriving in 25°W decreases in regions north of 50°N, while it increases at lower latitudes, around 46°N. This pattern may suggest a barrier to dispersal towards northern areas, such as Denmark while potentially facilitating population growth in southern regions, such as Portugal (manuscript III).

These results offer new insight on the ecology of those species, but they also highlight the importance of laboratory experiments to provide parameters fundamental to perform this type of individual-based models. Additionally, future research should also support the creation of open-access databases providing long-term information on the abundance of eggs and larvae, which can provide information to support model design and validation.