Mechanisms regulating inter‐annual variability in zooplankton advection over the Lofoten shelf, implications for cod larvae survival

Abstract

Variability in the availability of suitable prey (mainly nauplii stages of Calanus spp.) in the early life stages of cod is likely a significant contributor to the fluctuations in the year‐class strength in the Arcto Norwegian cod stock. We have investigated the origin of the Lofoten shelf population of Calanus using a particle tracking model approach based on velocity fields simulated from 2002 to 2012. By performing backwards simulations of trajectories from particles released on the Lofoten shelf in early spring, we assessed the relative contribution of potential Calanus sources, such as the Lofoten Basin, the local overwintering on the continental shelf and the adjacent fjords. This analysis revealed significant interannual differences in advection patterns mainly driven by changes in wind conditions and variable strength of the Norwegian Coastal Current. Most of the particles advected into our study area originated on the shelf, and contrary to expectations, only a few originated from the Lofoten Basin where Calanus abundances tend to be high. These results suggest that contribution from the Lofoten Basin to the early spring shelf Calanus population may be overestimated and that contribution from more local sources, especially from adjacent fjords, may play a more important role than what has been previously anticipated. Our results highlight the necessity to improve quantification of shelf‐fjord exchange processes, as these may contribute more to the regulation of spring zooplankton stocks on the northern Norwegian shelf, and, as a result, may affect the survival of cod larvae in this area.

Environmental effects on zooplankton abundance on a sub-Arctic shelf off northern Norway

“…This information may prove useful in the design of field studies and provide a base-line for validation of spatially resolved biological models (e.g. Espinasse et al 2017). We suggest the observed mesoscale patterns in abundance of important zooplankton species mainly reflected general ecological differences in numerical responses to environmental swings and in vertical habitat choice that seems to exist between the mainly herbivorous and omnivorous or carnivorous species (Bandara et al 2016).…”

“…The volume transport in the dominant current system of the study area tends to slow down and stabilize in spring and summer, primarily due to seasonality in wind regime and freshwater influence (Skagseth et al 2011). Although inshore Ekman transport in spring varies substantially between years, the study area is mainly fed by waters originating from coastal sources south of the study area and only to a limited degree from the oceanic region (Espinasse et al 2017; Fig. 3).…”

Phylogeography of the temperate marine bivalveCerastoderma edule(Linnaeus, 1758) (Bivalvia: Cardiidae) in the Subarctic: Unique diversity and strong population structuring at different spatial scales

“…The large‐scale water circulation near Lofoten provides a plausible explanation for the high haplotype diversity in Salangen (SAL, Figure ), and for the abrupt genetic change observed in the Barents Sea C. edule . Statistical modelling (Espinasse et al., ; Ommundsen, ) shows that larvae‐sized particles hypothetically released at Lofoten could be transported southwards. We suppose that the Maelström could be seen as a “trap” for the planktonic larvae drifting from the south.…”

“…The warm Norwegian Current brings nutrient-rich water, zooplankton, eggs and larvae of marine fishes and invertebrates along the Norwegian coast as north as to the Barents Sea (Gjøsaeeter, Bogstad, & Tjelmeland, 2009;Helle & Pennington, 1999;Mileikovsky, 1968 (7) Western Norway (1) Central Norway (2) (Espinasse et al, 2017;Ommundsen, 2002) shows that larvae-sized particles hypothetically released at Lofoten could be transported southwards. We suppose that the Maelstr€ om could be seen as a "trap" for the planktonic larvae drifting from the south.…”

Shift in the larval phenology of a marine ectotherm due to ocean warming with consequences for larval transport

“…Along with biological factors (hatching time, larval duration, and behavior), transport in the pelagic larval stages is dependent on physical factors such as ocean circulation patterns which will affect recruitment to the adult population (Ospina‐Alvarez et al 2018; Blanco et al 2019). Considerable interannual variability can arise in oceanographic transport patterns (Espinasse et al 2017), therefore we need to understand how important this interannual variation is for larval retention within grounds/habitat, or exports of larvae to neighboring grounds. A spatial influence according to release location may also occur, if the grounds where larvae are produced is large, larvae being released from one area may have a higher likelihood of retention than those from another area.…”