Length-related mortality and recruitment in cod ( Gadus morhua )

In 2007, the harvest control rule of the Icelandic cod stock was changed from 25% to 20% harvest rate per year. This change promoted a large increase in the abundance of older cod. Subsequently, both the spawning stock biomass (SSB) and the recruitment of 1-year-old cod increased. However, recruitment of 3-year-old cod showed no corresponding increase. Our main task was to study the mortality of cod from age 1 to age 3. We formulate natural mortality (M) as M = ML + MC, where ML is length-related mortality and MC condition-related mortality, which in this study is assumed to be constant (MC = 0.2). Due to size-dependent mortality, ML is assumed to decline exponentially with fish length (L), ML = βe−kL, approaching 0 above age 3. The data analysis is mainly based on two annual groundfish surveys in March 1985–2024 and October 1996–2024. During the study, the mean lengths of 1-year-old cod decreased and thus our estimates of ML increased. Stomach content analysis indicated that cod may be the most important demersal predator of cod and that the average number of cannibalized cod per older cod increases with age of cannibals. Thus, with increasing SSB, total cannibalism increased. Relative mortality indices, RZage1 to age3, based on survey abundance indices, were studied in relation to two environmental factors, the Atlantic Multidecadal Oscillation (AMO) and pre-bloom silicic acid concentration (Si), and two stock-related factors, mean length of 1–2-year-old cod (Lmean) and SSB. The best model in a time series analysis indicated that RZ was affected negatively by both Lmean and Si. The analysis suggests that a reduction in pre-bloom Si may have reduced growth rate and survival of cod in their first year (bottom-up effects). Additionally, the increased abundance of older cod during the study period may have increased cannibalism on 1-to-3-year-old cod (top-down effects).