Sudden change in long‐term ocean climate fluctuations corresponds with ecosystem alterations and reduced recruitment in Norwegian spring‐spawning herring (Clupea harengus, Clupeidae)

Abstract

Fish stocks vary in abundance. The causes behind the fluctuations may be difficult to determine, especially ones caused by natural fluctuations, but long‐term data series may provide indications of the mechanisms. Assessments show that the recruitment to the Norwegian spring‐spawning herring (Clupea harengus, Clupeidae) has remained low since 2004, a year which produced the last really rich year‐class. Long time‐series of estimated recruitment and mean winter temperature in the ocean showed a significant positive correlation for the period 1921–2004. Here, we show that this positive correlation did not continue from 2005 onwards as the winter temperature increased to high levels while herring recruitment decreased and has remained low. The density of zooplankton in the drift route of the herring larvae dropped significantly after 2004, and their centre of gravity shifted northwards. There may currently be heavy predation on the larvae by Atlanic mackerel (Scomber scombrus, Scombridae), and top‐down regulation is suggested to hamper successful recruitment. Our analysis indicates that the presence of food and overlap with high food concentrations are likely important regulators of survival in herring larvae. The findings may be important for future management and planning of fisheries of this stock because recruitment failure may continue if temperature remains high and food abundance remains low.

Changing how we approach fisheries: A first attempt at an operational framework for ecosystem approaches to fisheries management

“…Another important diagnostic population feature has been a long‐term reduction in length‐at‐age (Table ; Huse et al, ; ICES ). The decrease in zooplankton abundance (Dupont, Bagøien, & Melle, ; Huse et al, ; ICES, ; Toresen et al, ) is further evidence that also supports this possible mechanism. Our proposed framework necessitates stepping through higher risk mechanisms first, but still allows for the exploration of all possible mechanisms.…”

“…Some other stocks, including the Norwegian Spring Spawning (NSS) Atlantic herring ( Clupea harengus , Clupeidae), have conversely not produced a strong year class since 2004 (ICES, ). This might be related to the climatic condition or to the reduced zooplankton abundance seen both in the Norwegian Sea (Huse et al, ) and along the coast (Toresen, Skjoldal, Vikelao, & Martinussen, ), but the causes for the poor recruitment are far from being fully understood (ICES, ). The blue whiting ( Micromesistius poutassou , Gadidae) has had some fluctuations in abundance in last the 30 years with strong variation both in recruitment and in adult abundance.…”

“…The diagnostics for NSS herring are less clear (Tables and ). The most pronounced pattern in recent years has been a decline in stock size, mainly in relation to poor recruitment since 2004 (Skagseth, Slotte, Stenevik, & Nash, ; Toresen et al, ). The mechanisms give a lot of similar signs for recruitment, biomass and abundance.…”

“…Some other stocks, including the Norwegian Spring Spawning (NSS) Atlantic herring (Clupea harengus, Clupeidae), have conversely not produced a strong year class since 2004 (ICES, 2018). This might be related to the climatic condition or to the reduced zooplankton abundance seen both in the Norwegian Sea and along the coast (Toresen, Skjoldal, Vikelao, & Martinussen, 2019), but the causes for the poor recruitment are far from being…”

An appraisal of the drivers of Norwegian spring‐spawning herring (Clupea harengus) recruitment

“…Historically, a number of hypotheses related to processes occurring during the first year of life of herring were tested using abundance estimates derived from stock assessments (Toresen & Østvedt, 2000; Toresen et al., 2019). Because they were derived from cohort analysis (VPA), these estimations of abundance are based on observed abundances at a later age and are estimated by assuming constant mortality at young ages ( M = 0.9 for age 0–2, M = 0.15 for age 3 and older).…”