Seasonal changes in clupeids maturation constrains the food quality of chicks of the common guillemot (Uria aalge) : A case study of a potential mismatch, from the perspective of the common guillemot chicks on Stora Karlsö, the Baltic Sea

Abstract: Background: Common guillemots are integral elements of the Baltic Sea marine ecosystem. They possess life-history characteristics such as relatively long lifespan, quite low fecundity resulting in only one egg per breeding season – characteristics that make them particularly vulnerable to even small changes in the environment. Higher weight of their single chick, gained during the breeding season, ensures higher survival rates for Guillemot fledglings. Hence, during the breeding season not only quantity but also quality of prey within the foraging area are central for their reproduction success. Objectives: This study applies the match-mismatch hypothesis on a predator-prey relationship by investigating common guillemot chicks’ fledgling weight on Stora Karlsö in relation to their key prey, sprat and herring. Maturity in sprat and herring were used as an indicator for fitness and spawning abundance of food quality for parental guillemot for raising chicks’.   Data: The study used data of chicks’ weights when leaving their nests from common guillemots (40,936 chicks) collected between 2007 and 2017 from the Swedish Baltic Sea Bird Project. Data of sprat and herring gonadal maturity was obtained from the PLANFISH project and inspections of catches from the commercial and industrial fisheries.   Method: Common guillemot chick fledgling weights between years were analysed by Analysis of Variance (ANOVA). Maturity in sprat and herring were used as a proxy to estimate spawning state and predict the peak day of highest abundance of potential spawners. A Generalized Additive Model (GAM) was used to predict the peak day of highest abundance of potential spawners. For estimating match-mismatch lags, a cross-correlation function analysis (CCF) was conducted to analyse chick fledgling weight in relation to sprat and herring’s maturity states (fitness).   Results: The study showed that common guillemot chicks have decreased in weight annually between 2002 and 2017. The results further show that it is obviously more advantageous for chicks to leave their breeding ledge during the first part of the fledgling period (end of June) since chicks that leave their breeding ledge during the end (mid July) of the period showed a lower mean weight. The statistical analysis also showed that chicks had a significant weight loss of approximately 3 % between all pairwise compared consecutive years (2009-2017). The analysis of sprat and herring maturity (fitness) resulted in strong inter-annual variation, and analysis showed that intra-annual fish maturity has an influence on chick weight. Cross correlation analysis revealed a significant positive correlation lag between herring mean maturity and chick mean weight during the fledgling period at day 0-2 days before leaving nest and a significant negative correlation at lag days 3-20 (approximate time period of hatching to young chick) but a negative correlation between sprat mean maturity and chick fledgling weight at lag 5-9 days after leaving nest and a significantly positive correlation between day 9-19 (approximate date of hatching). These results indicate that sprat might be the essential and necessary food during the chicks’ first period while herring comes to play a more vital role in the later.   Conclusion: This study shows that breeding success in common guillemots not only strongly depends on quantity of essential fish prey species but also on the food quality (fitness) of the fish prey species. Moreover, chicks’ weight, and thus their potential later survival, is strongly dependent on the right timing of abundance of the developed maturity stages of the two relevant fish species, sprat and herring, during the 21 days of the breeding season. The study thus helps to clarify final causes and consequences of seasonal phenological changes in species’ lifehistory traits and the effects on other species.