Question: Atlantic cod fish, according to the NEFSC status of the ecosystem report, show low weight at age, low length at age, low fecundity and low recruitment. What could be the cause of that?
The primary causes of low weight-at-age, low length-at-age, low fecundity, and low recruitment in Atlantic cod (Gadus morhua) in the Northeast U.S. (particularly Gulf of Maine and Georges Bank stocks) are ecosystem and climate-related drivers identified in NEFSC analyses tied to the Status of the Ecosystem reporting framework. (LINK)
These patterns are documented in the Atlantic cod Ecosystem and Socioeconomic Profile (ESP), developed as part of NEFSC’s next-generation stock assessment tools. The ESP explicitly links cod recruitment (age-1 abundance and recruits per spawner), fish condition, weight-at-age, and related metrics (such as depth/latitude of catch) to four key environmental indicators:
– Sea surface and bottom temperature anomalies.
– Gulf Stream Index (a proxy for broader oceanographic shifts and warm-water influence on the Northeast Shelf).
– Zooplankton abundance anomalies, especially Calanus finmarchicus and Pseudocalanus spp.
– Mean cumulative marine heatwave index. (LINK)
How These Drivers Produce the Observed Symptoms
Here’s a breakdown of the mechanisms, based on NEFSC-linked assessments and supporting cod biology:
1. Ocean warming and temperature anomalies (including marine heatwaves)
Atlantic cod are a cold-water species with optimal thermal ranges for growth, spawning, egg development, and larval survival. Rapid warming in the Gulf of Maine (one of the fastest-warming ocean regions) and frequent marine heatwaves increase metabolic costs while reducing growth efficiency. This directly causes low length-at-age and low weight-at-age (poorer somatic growth and condition).
Warmer temperatures also shift spawning timing/location, reduce egg/larval survival, and contribute to low recruitment. Heatwaves amplify these effects through cumulative stress. (LINK) (LINK)
2. Declines in key zooplankton prey (Calanus finmarchicus and Pseudocalanus spp.)
These copepods are critical food for cod larvae and early juveniles. Lower abundances create a “match-mismatch” where larvae have insufficient prey at first-feeding stages, leading to starvation, slower growth, and high mortality. This drives low recruitment.
Reduced prey availability throughout the food web also contributes to poor adult/juvenile condition(low weight- and length-at-age), as cod have less energy for growth. (LINK) (LINK)
3. Broader oceanographic changes (Gulf Stream Index)
Shifts in Gulf Stream position bring warmer, more stratified waters onto the shelf, altering productivity, prey fields, and habitat suitability. This reinforces the temperature and zooplankton effects above, affecting distribution (deeper or more northern catch) as well as growth and survival metrics. (LINK)
4. Link to low fecundity
Fecundity (eggs per female) in Atlantic cod is strongly size- and condition-dependent—larger, older, better-conditioned females produce far more and higher-quality eggs. Poor growth (low length/weight-at-age) and low condition mean fish reach smaller sizes at a given age and allocate less energy to reproduction. This is compounded by maternal-age effects: stocks with truncated age structures (fewer large/old females) have inherently lower reproductive output per spawner, feeding into low recruitment. (LINK)
Additional Contributing Factor: Legacy/ongoing fishing pressure
Many cod stocks (e.g., western Gulf of Maine) remain overfished, with overfishing still occurring in some cases. Size-selective fishing has historically truncated the age structure, removing the largest, most fecund females. While current low biomass should theoretically reduce density-dependent competition and improve growth, the dominant environmental drivers (warming + prey declines) override this benefit, keeping condition and recruitment poor.
These ecosystem drivers are the focus of NEFSC’s recent cod research-track assessments and ESP work, which incorporate them to better explain why rebuilding has been slower than expected despite reduced fishing mortality. In short, the combination of a warming, changing prey environment is creating suboptimal conditions across cod life stages—poorer growth/condition in juveniles and adults, reduced egg production, and low survival to the recruited stage. Ongoing monitoring through the Status of the Ecosystem reports and ESPs tracks these indicators to inform management.