Latitudinally distinct stocks of Atlantic cod face fundamentally different biophysical challenges under on‐going climate change

Abstract

The reproductive success of marine ectotherms is especially vulnerable in warming oceans due to alterations in adult physiology, as well as embryonic and larval survival prospects. These vital responses may, however, differ considerably across the species' geographical distribution. Here we investigated the life history, focusing on reproductive ecology, of three spatially distant populations (stocks) of Atlantic cod (Gadus morhua, Gadidae) (50-80° N), in the Irish/Celtic Seas-English Channel Complex, North and Barents Seas, under past and projected climate. First, experimental tracking of spawning behaviour evidenced that the ovulation cycle is highly distressed at ≥9.6 (±0.25)°C (T up ). This knife-edge threshold resulted in erratic spawning frequencies, whereas vitellogenin sequestration remained unaffected, indicating endocrine rather than aerobic scope constraints. Cod in the Celtic Sea-English Channel are, therefore, expected to show critical stock depensation over the next decades as spawning grounds warm above T up , with Irish Sea cod subsequently at risk. Second, in the relatively cooler North Sea, the northward retraction of Calanus finmarchicus (Calanidae)and Para-Pseudocalanus spp. (Clausocalanidae) (1958-2017) limit cod larvae feeding opportunities, particularly in the southernmost subarea. However, the contrasting increase in Calanus helgolandicus (Calanidae) does not counteract this negative effect, likely because cod larvae hatch ahead of its abundance peaks. Overfishing again comes as a twin effect. Third, in the still relatively cold Barents Sea, the sustainably harvested cod benefit from improved food conditions in the recent ice-free polar region but at the energetic cost of lengthier and faster spawning migrations. Consequently, under climate change local stocks are stressed by different mechanistic factors of varying management severity.

Spawning fish maintains trophic synchrony across time and space beyond thermal drivers

“…Note that this equation fails above 9.6 C, that is, when reproductive problems (like lower egg fertilization rates) start to occur in this species in too warm waters (van der Meeren & Ivannikov, 2006). Thus, the threshold 9.6 C refers to the upper pejus temperature for ovulation (Kjesbu et al, 2023). Using Equation ( 2), we back-calculated the time it takes for the first group of oocytes (leading cohort, LC) to grow backwards from 690 μm, which is considered to be shortly before oocyte hydration (swelling) and first spawning, to a minimum diameter of 250 μm, reflecting the initiation of the reproductive cycle (vitellogenesis) (Kjesbu et al, 2010).…”

“…Atlantic cod, Gadus morhua, has been a key species in the development of the match-mismatch hypothesis, as they spawn around the time of the spring bloom and the stocks have undergone significant changes in recruitment linked to climate change (Beaugrand et al, 2003;Kjesbu et al, 2023;Kristiansen et al, 2011). Originally, the match-mismatch hypothesis implicitly assumes a near-constant fish spawning time across years and where variable timing of food availability for the fish larvae is driven by interannual fluctuations in phytoplankton bloom timing (Cushing, 1969(Cushing, , 1990.…”

Recruitment regime shifts and nonstationarity are widespread phenomena in harvestable stocks experiencing pronounced climate fluctuations

“…However, for NEA cod the updated GAM revealed a significant concave pattern, where the turning point was around 10 times more capelin than NEA cod SSB. More in‐depth explorations show that the negative trend seen to the left in this curve principally refer to data after 2005, suggesting that the recent higher migration costs, due to a more northward displacement in the Barents Sea (Kjesbu, Alix, et al., 2023), might have negatively affected the fecundity (dos Santos Schmidt et al., 2017). Therefore, the current version of food availability for the adults of NEA cod lacks the spatial perspective.…”

“…Our meta‐analysis supports the notion that a higher T is expected to improve the recruitment strength of commercial stocks located in the higher‐latitude Barents Sea (Bogstad et al., 2013; Øiestad, 1994). The opposite is the case in the lower‐latitude North Sea (Kjesbu, Alix, et al., 2023) but in that ocean basin also observing smaller body sizes (cf. Bergmann's rule) (Baudron et al., 2014).…”

“…The time series of Hylen (2002), however, does not have any estimation of uncertainty which limits its usefulness in certain model situations (see below). Annual mean temperatures at 0–200 m depth in the Kola Section (Kola temperature) – running along the 33°30′ E meridian from 70°30′ to 72°30′ N and covering the period from 1921 to 2020 – were extracted as an index for the long‐term thermal change in the Barents Sea (Boitsov et al., 2012; Kjesbu, Alix, et al., 2023). Winter AMO and NAO from 1921 to 2021 were added to this data inventory to represent large‐scale climate variability (Section 2.2.3).…”