Age validation of age 0-3 wild cod Gadus morhua in the western Baltic Sea through mark-recapture and tetracycline marking of otoliths

Abstract

<jats:p>The contrasting zones within otoliths are commonly used to infer age of fish, assuming that 1 opaque zone and 1 translucent zone (TZ) form per year. However, the periodicity of zone formation is often un-validated and susceptible to interpretation errors and uncertainties, which propagate into age-structured calculations. The western Baltic cod <jats:italic>Gadus morhua</jats:italic> (WBC) stock is currently evaluated through an age-based assessment, using the typical, temperate area assumption that TZs form in autumn/winter. The periodicity of zone formation was evaluated through a mark-recapture experiment with juvenile and young adult WBC. During 2014-2017, 9111 wild cod were tagged externally (T-bar anchor tag) and internally (injection with tetracycline-hydrochloride induced a permanent fluorescent mark on their otoliths) and released. Timing of TZ formation was reconstructed through examination of otoliths from recaptured age 0-3 cod for which all required information and otoliths were available (n = 59, length range: 173-500 mm). Contrary to the assumption that TZs are formed during winter, recaptured cod consistently formed 1 TZ per year, between summer and autumn, in both juveniles (age 0-1) and young adults (age 2-3). A calendar year in the otolith is therefore composed of a TZ with opaque material on either side. Coincidence of TZ formation and recent evidence of shallow-water avoidance and reduced feeding and growth in WBC during peak summer suggests that TZs are formed when metabolic demands cannot be met. The results improve accuracy of WBC age estimates and highlight the feasibility and importance of conducting age validation experiments.</jats:p>

Is analysis of otolith microstructure a valid method for investigating early life history of Western Baltic cod?

“…The present study independently and directly confirmed that only one TZ in WBC otoliths is formed in the first year, and that it is formed during the peak summer period. These findings are in line with recent age validation studies of cod from the same region (McQueen et al 2019;Krumme et al 2020). We can specify that the TZ formation in age-0 cod coincided with the period when ambient water temperatures exceeded optimal water temperatures for growth of cod of this size (~ 17-19 °C, calculated from Björnsson (2001)).…”

“…The Stollergrund is an abrasion platform just north of the Kiel Bight and is relatively exposed compared to the southern coast of Fehmarn, where seagrass beds provide shelter for juvenile cod (Krumme et al 2020). YOY cod in the Kiel Bight were likely spawned in the small Belt (Thurow 1963), while juveniles at Fehmarn were probably spawned in the Fehmarn Belt or Mecklenburg Bight (Bagge et al 1994;Brander 2005).…”

“…A total of 13 tetracycline-marked recaptured juvenile cod from an age-validation experiment were available (Krumme et al 2020, Table 2). The otoliths were processed using the "embed and polish" method, and micro-increments were counted and measured along the dorso-ventral axis, as with the YOY otoliths.…”

“…The composed age (in years: Age years ) of recaptured individuals was estimated following the age reading guide of Krumme et al (2020) and compared to the total number of counted micro-increments. A composed age in days (Age days ) for each fish was calculated as…”

Tracing growth patterns in cod (Gadus morhua L.) using bioenergetic modelling

“…Furthermore, reliability and quality of growth estimates strongly depend on recapture rates, which often do not reach desirable levels. Recent tag‐recapture experiments on cod in the Western Baltic Sea, for example, resulted in recapture rates of less than 1%, highlighting the challenges of this approach (Krumme et al., 2020).…”