Table 1.
Sampling area and environmental and life history traits of the cod populations whose otoliths were used in this study.
Fig 1.
Comparison of spawning zone absence and presence in two NEA cod otoliths.
a) 10-year-old fish with no visible spawning zones; b) 10-year-old fish with five visible spawning zones, identified as a cluster of similar-looking narrow increments with significantly wider translucent zones. Black circles mark regular growth increments; white circles mark spawning zones as described above; semi-filled white circles indicate an incomplete marginal increment that would likely be a spawning zone when fully formed.
Table 2.
Age distribution of the otolith images read from each population.
Fig 2.
Comparison of width of increments formed at the same age and identified as either non-spawning or spawning zones.
An age-verified subsample of 8-years old NEA cod otoliths was used for the comparison.
Fig 3.
Annotated examples of otoliths with visible spawning zones.
NEA cod (a), NCC (b), NS cod (c), Icelandic cod (d), Greenland cod (e) and Cape Breton cod (f). Spawning zones shown as white circles.
Fig 4.
Proportions of otoliths and spawning zones identified per age class and population.
Color scale refers to the number of spawning zones counted. Column numbers indicate the number of otoliths available from each age class for each population. Dashed lines represent the proportions of mature at age calculated from the maturity ogives from stock assessment data for each population in the period of sample collection.
Fig 5.
Proportions of mature at age (colors) calculated for each population from the visual identification of individual spawning zones, compared to the mean proportions of mature at age calculated from the maturity ogives for the period specific to each population.
Horizontal bars indicate the standard deviation of the mean of each proportion mature at age over the range of years defining the samples. Point size is proportionate to the number of individual increments per age class.
Fig 6.
Maturity ogives of NEA cod extracted from stock assessment and reconstructed using individual spawning zones, aggregated by decade.
Fig 7.
Proportions of mature-at-age per year and age class from the official stock assessment ogives (black) and reconstructed for each year from individual increments identified as spawning zones (blue). Vertical dotted line (1959) represents the beginning of the period combining Norwegian spawning zone data and Russian gonad readings for calculating maturity at age in stock assessment; vertical dashed line (1982) represents the beginning of yearly Barents Sea and Lofoten bottom trawl surveys and the change to using only gonad maturity stages in stock assessment.
Fig 8.
a) Median and b) interquartile range of age at maturity of NEA cod between 1930 and 2015. Calculated from the latest maturity ogives available from stock assessment (black solid line and points) and the proportions of mature-at-age reconstructed for each year from individual increments identified as spawning zones from the whole archive (blue dashed line and open points). See Fig 7 for vertical lines (1959 and 1982).
Fig 9.
Time series of maturity-at-age derived from spawning zones and from gonads.
Mean proportions of NEA cod mature-at-age (colors) calculated from individual increments from the whole archive identified as spawning zones for each year, compared to the mean proportions of mature-at-age calculated from the maturity ogives from stock assessment and aggregated by decade (facet). Error bars indicate the standard deviation of maturity ogives from spawning zones (vertical) and stock assessment (horizontal) within each decade. Point size is proportional to the number of individual increments per age class.
Fig 10.
Comparison of changes in age at maturity between readings.
Mean age at maturity of fish captured at age 8 calculated for each year between 1932 and 2015 from the entire archive (black dashed line), the original readings of our subsample (green dashed line) and the reinterpretation by a single contemporary reader (solid blue line).
Fig 11.
Comparison of number of spawning zones between readers.
Yearly mean number of spawning zones in fish of age 8 calculated from the original and the reinterpreted subsample, decades coded by color.