Drag force and reconfiguration of cultivated Saccharina latissima in current

The design of aquaculture systems requires an understanding of the drag forces on cultivated kelp. This study measured the drag on line segments of cultivated Saccharina latissima in a towing tank. The drag on segments of farm line with full kelp bundles and with stipes alone (fronds removed) was measured at tow speeds of 0.10 to 0.50 m/s. The drag on individual fronds cut from the line was also measured. Video images were collected to evaluate the plant reconfiguration. Both kelp blades and stipes contributed to the total drag force on the line bundle. Within the velocity range of our experiments, the kelp blades were essentially horizontal. However, the pronation of kelp stipes increased as flow velocity increased. The reconfiguration of kelp stipes was observed to decrease the vertical extent of the kelp bundle. Due to this reconfiguration, the measured force, $F$, increased with velocity, $U$, at a rate slower than quadratic, and was consistent with scaling laws derived for reconfiguration. Specifically, $F\sim U^{\alpha }$ with $\alpha =1.35\pm 0.17$.     

Reproductive resilience or sweepstakes recruitment? Assessing drivers of lifetime reproductive success in exploited marine fish

Understanding the processes that drive reproductive success in marine fish stocks is critical to effective fisheries management. These processes can be difficult to investigate, especially in age-structured populations, because they occur at transgenerational scales. Reproductive success is often attributed to a small portion of the adult population (<0.01%) and thought to be driven primarily by random external factors, consistent with the concept of sweepstake reproductive success (SRS). A competing concept, the reproductive resilience paradigm, posits that fishes have evolved complex spawner-recruit systems to achieve lifetime reproductive success and maintain population stability within highly variable environments. Here, we examine these two concepts. First, we analyse the popular sport fish red drum (Sciaenops ocellatus, Sciaenidae), drawing on genetic and reproductive data to estimate a plausible range for the ratio of effective population size ($N_e$) to adult abundance ($N_A$) and to infer variance in lifetime reproductive success ($V_k^{*}$). Then, we synthesize available data and infer $V_k^{*}$ for two other fishes that have $N_e/N_A$ ratios reportedly >0.10, the southern bluefin tuna (Thunnus maccoyii, Scombridae) and the silver seabream (Chrysophrys auratus, Sparidae). Although commonly regarded as an SRS species, red drum did not meet the SRS criterion. Overdispersed $V_k^{*}$ values were inferred for all three species, with those for red drum and silver seabream being dependent upon population-closure assumptions. Results are presented within the conceptual framework of reproductive resilience, considering the roles of random extrinsic forces versus evolved traits to achieve lifetime reproductive success and population stability in high and variable mortality environments.