DOI: 10.5061/dryad.t4b8gtj4s
title: Otolith annual growth increments for cod populations in the Northeast Atlantic

description: Large-scale, climate-induced synchrony in the productivity of fish
 populations is becoming more pronounced in the world’s oceans. As
 synchrony increases, a population’s ‘portfolio’ of responses can be
 diminished, in turn reducing its resilience to strong perturbation. Here
 we argue that the costs and benefits of trait synchronization, such as the
 expression of growth rate are context dependent. Synchrony among
 individuals could actually be beneficial for populations if growth is
 optimized during favourable conditions and then declines under poor
 conditions when a broader portfolio of responses is needed. Importantly,
 growth synchrony among individuals within populations has seldom been
 measured, despite well-documented evidence of synchrony across
 populations. Here, we used century-scale time series of annual otolith
 growth to test for changes in growth synchronization among individuals
 within multiple populations of a marine keystone species (Atlantic cod,
 Gadus morhua). On the basis of 74,662 annual growth increments recorded in
 13,749 otoliths, we detected a rising conformity in long-term growth rates
 within northeast Atlantic cod populations in response to both favorable
 growth conditions and a large-scale, multidecadal mode of climate
 variability similar to the East Atlantic Pattern. The within-population
 synchrony was distinct from the across-population synchrony commonly
 reported for large-scale environmental drivers.  Climate-linked,
 among-individual growth synchrony was also identified in other Northeast
 Atlantic pelagic, deep-sea and bivalve species. We hypothesize that growth
 synchrony buffers marine populations to changing climate and growth
 conditions through its effect on the phenotypic expression of growth
 diversity, and thus provides an unexpected, but pervasive and stabilizing
 impact on marine population productivity.
description: Growth chronologies were based on cod sampled at annual intervals
 over periods of up to 94 years from five major cod populations in the
 Northeast Atlantic (Table S1). For the migratory populations of Norway and
 Iceland, samples were collected from the main spawning grounds during the
 spawning season (Norway: the Lofoten archipelago, January - early May;
 southwestern Iceland: March – May). The Faroe cod population was sampled
 on the Faroe plateau spawning grounds during the spawning season (February
 – April) at bottom depths shallower than 150 m. The Godthaabsfjord cod
 population on the west coast of Greenland (64°N, 51°W, NAFO Division 1D)
 was sampled mainly (88%) between April and September, with small numbers
 caught during the reminder of the year. Cod from the inshore area around
 Sisimiut, West Greenland (66°45’N, 53°30’W, NAFO Division 1B) were
 primarily caught during June to August (70%), whereas the rest were caught
 during April, May, September and October. Most samples were collected with
 research or commercial bottom trawls, supplemented by commercial
 longlines, jigs, and pound nets. Otoliths from the above samples were
 subsequently retrieved from otolith archives at the Faroese Marine
 Research Institute (Faroe Islands), Greenland Institute for Natural
 Resources (Greenland), Marine and Freshwater Research Institute (Iceland),
 and Institute of Marine Research (Norway). Due to a probable
 size-selectivity bias, otoliths from fish caught using gillnets were
 excluded from the Icelandic and Norwegian selection (Smoliński et al.
 2020a, Denechaud et al. 2020). In order to robustly
 estimate growth variation across growth years and annual fish cohorts,
 large sample sizes from multiple overlapping cohorts are required
 (Morrongiello et al. 2012, Smoliński et al. 2020b). Wherever possible,
 samples for the Icelandic and North East Arctic (NEA) cod populations
 consisted of at least 50 otoliths per year from mature fish (age 8 or
 older), although the sampling target was 30 otoliths per year for the
 Faroese (ages 5-6), Godthaabsfjord (ages 5-6), and Sisimiut (ages 4-10)
 populations. The accuracy of the age determinations was high (Smoliński et
 al. 2020a). Otolith growth chronologies
 Otolith growth chronologies were constructed from series of
 annual increment widths measured from digitized images of sectioned
 otoliths. Since the date and age at capture (corresponding to the otolith
 margin) was known, each increment could be assigned a year and age of
 formation. Norwegian, Icelandic, and Faroese (1980-1990 only) otoliths
 were embedded in epoxy and sectioned transversely through the core
 (Smoliński et al. 2020a, Denechaud et al. 2020). The Godthaabsfjord,
 Sisimiut, and post-1990 Faroese otoliths were sectioned without embedding
 and subsequently heat-treated to increase the contrast between opaque and
 translucent zones (Christiensen 1964). All images were captured under
 reflected light using high-resolution image analysis systems. Increment
 widths (μm) were measured along an axis drawn from the otolith core to the
 distal edge, thus intersecting the maximum number of annual increments at
 a perpendicular angle (Figure 1A). In Norway and Iceland, because the
 position of the core was not always clear, the longest diameter of the
 first increment was marked and the intersection point between the diameter
 and the measuring axis was used as the origin for the measurements
 (Denechaud et al. 2020). Because of this difference, the width of the
 innermost increment was not included in the analysis of the Icelandic
 samples. Annual increments were measured as the width of a translucent and
 opaque zone pair: from the medial edge (distal edge in the case of Iceland
 and NEA) of the opaque zone to the end of the subsequent translucent zone,
 and were measured across the entire growth sequence of each otolith. Here,
 the data analysis was restricted to increments formed at ages 1 to 6,
 since these ages were represented in all populations and most fish were
 still sexually immature such that inter-annual growth fluctuations more
 likely reflected environmental conditions and/ or population abundance
 rather than the energetic costs of reproduction.