Mohamed Yosri Zanni

Accurate interpretation of marine environmental DNA (eDNA) monitoring surveys depends on understanding how eDNA concentrations change once released by organisms into the surrounding environment. Despite growing evidence for multiple eDNA decay patterns, such as biphasic and delayed decay, many studies assume by default a single‐phase exponential decay model. Here, we took a multiprong approach with a literature review of eDNA decay experiments, an experimental study of empirical decay patterns, and high‐resolution Lagrangian particle tracking to quantify how different decay patterns drive eDNA dispersal at two contrasted locations in the Bay of Biscay, Northeast Atlantic. Biphasic or delayed eDNA dynamic patterns were found in over half of the reviewed empirical studies and at 13°C and 20°C in our experiment, with the first 12 h being key to detecting biphasic or delayed decay dynamics. Compared to single‐phase exponential decay, the alternative decay dynamics increased eDNA concentrations by about a factor of two to four after 24 h, depending on the decay pattern. Assuming an exponential decay may lead to under‐ or overestimation of the amount of eDNA released when the decay dynamics are actually biphasic or delayed, depending on local hydrodynamics. In conclusion, considering the appropriate eDNA decay type is critical for marine eDNA transport models and eDNA monitoring data interpretation.

There is increasing interest in using eDNA for deriving abundance indices for biodiversity monitoring and in support of fisheries management. However, eDNA concentrations are affected by animal behavior, such as diel vertical migration, which has repercussions for designing eDNA sampling strategies for deriving unbiased abundance indices. In this study, we investigated the potential impact of diel vertical migration or other diel activity variations on measured eDNA concentrations for European hake ( Merluccius merluccius ), European seabass ( Dicentrarchus labrax ) and blackspot seabream ( Pagellus bogaraveo ). For hake, in situ eDNA concentrations near the sea floor differed systematically between samples taken before sunrise and after sunset, with the average concentration in morning samples being 24% of the average evening samples. For the two other species, only a weak diel signal in eDNA concentrations was found. Modeling the dispersal and decay of eDNA particles through a Lagrangian approach revealed that eDNA concentrations might decrease to 21%–41% of their initial value during the absence of a species moving entirely up from the sea floor during the night. For M. merluccius , the coherence between observed diel variations in eDNA concentrations near the sea floor and modeling results indicates that diel vertical migration behavior needs to be accounted for when devising eDNA sampling plans. The necessity is less clear for D. labrax and P. bogaraveo .

Environmental DNA (eDNA) has emerged as a powerful tool for fisheries management and biodiversity monitoring, offering novel insights into marine ecosystems. However, linking eDNA concentrations to species abundance remains a significant challenge. Limited understanding of the biotic and abiotic factors influencing eDNA production, decay, and transport in marine environments continues to hinder its broader application. This study aims to address these gaps by modeling eDNA decay and transport dynamics using a Lagrangian particle tracking model, the Connectivity Modeling System. Specifically, we (1) fitted and implemented five temperature‐dependent decay rate relationships and (2) simulated eDNA transport accounting for temperature‐dependent decay rate variability. We modeled eDNA dispersal at three contrasting locations in the Bay of Biscay, Northeast Atlantic, over a full year under the five decay rate scenarios. For eDNA transport, current velocity was the most important factor, followed by the decay rate relationship, while the converse effects were found for eDNA dispersion and lifetime. Temperature was found to have the least impact on transport variability. On average, eDNA persisted between 5 and 30 h, with transport distances varying between 0.3 km and 39.1 km, depending on location, month, decay rate, and depth. Our results emphasize the need for precise decay rate estimates, tailored to species‐specific and encountered temperature conditions, to enhance the power of eDNA‐based monitoring.

Our general purpose is to support the use of known-biomass production models (KBPMs), illustrating their usefulness by addressing the evolution of surplus production (SP) over time and the factors affecting it (e.g. environment). We also demonstrate the utility of KBPMs for multispecies management objectives or for estimating maximum sustainable yield reference points without a stock recruitment function, among other worthwhile applications. To do so, we present different uses of KBPMs, illustrating their application on demersal species in the International Council for the Exploration of the Sea (ICES) area, specifically for megrim, white anglerfish and European hake stocks. The proposed analytical approach involved fitting single-species and multispecies KBPMs, conducting retrospective analyses and assessing the effects of environmental variability on SP. The findings show that, in general, stock SP increased after a decline in biomass and SP, except for white anglerfish in the southern area. Megrim stocks are the least productive, while hake and northern anglerfish are the most productive. Retrospective analysis revealed SP shifts in northern hake stock for reasons other than biomass variability. Hence, the North Atlantic Oscillation and the Atlantic Multidecadal Oscillation (AMO), two key climate variability modes in the North Atlantic, were tested for their links to SP, revealing a positive connection between SP and AMO, although further research is necessary. Beyond the specific results of our particular KBPM application, our main conclusion is that KBPMs can serve as a tool complementary to more complex assessment models for resolving unaddressed issues and crosschecking available assessment results.