Mechanisms of long-term decline in size of lesser sandeels in the North Sea explored using a growth and phenology model

Abstract

The lesser sandeel Ammodytes marinus is a key species in the North Sea ecosystem, transferring energy from planktonic producers to top predators. Previous studies have shown a longterm decline in the size of 0-group sandeels in the western North Sea, but they were unable to pinpoint the mechanism (later hatching, slower growth or changes in size-dependent mortality) or cause. To investigate the first 2 possibilities we combined 2 independent time series of sandeel size, namely data from chick-feeding Atlantic puffins Fratercula arctica and from the Continuous Plankton Recorder (CPR), in a novel statistical model implemented using Markov Chain Monte Carlo (MCMC). The model estimated annual mean length on 1 July, as well as hatching date and growth rate for sandeels from 1973 to 2006. Mean length-at-date declined by 22% over this period, corresponding to a 60% decrease in energy content, with a sharper decline since 2002. Up to the mid-1990s, the decline was associated with a trend towards later hatching. Subsequently, hatching became earlier again, and the continued trend towards smaller size appears to have been driven by lower growth rates, particularly in the most recent years, although we could not rule out changes in size-dependent mortality. Our findings point to major changes in key aspects of sandeel life history, which we consider are most likely due to direct and indirect temperature-related changes over a range of biotic factors, including the seasonal distribution of copepods and intra-and inter-specific competition with planktivorous fish. The results have implications both for the many predators of sandeels and for age and size of maturation in this aggregation of North Sea sandeels. KEY WORDS: Ammodytes marinus · Growth model · MCMC · North Sea · Phenology · SandeelResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 432: 137-147, 2011 138 dynamics and to impact higher trophic levels, and indeed have already been linked with fluctuations in both commercially exploited fish stocks , Castonguay et al. 2008 and reproductive performance of land-based predators (Frederiksen et al. 2006, Sydeman et al. 2006.The lesser sandeel Ammodytes marinus (hereinafter sandeel) is a key species in the ecology of the North Sea and other parts of the NE Atlantic (Daan et al. 1990). It is generally the most abundant small planktivorous fish, and provides a critical trophic link between secondary producers (zooplankton, particularly calanoid copepods) and a wide variety of top predators, including larger fish, marine mammals and seabirds. Several studies have highlighted the critical importance of the availability and quality of sandeels for successful reproduction of North Sea seabirds (Monaghan 1992, Rindorf et al. 2000, Wanless et al. 2005. Sandeels are also the target of a major industrial fishery for fishmeal and fish oil (Furness 2002). However, evidence from several sources has indicated that conditions for sandeels in the North Sea are less favourable than...

Effects of copepod size on fish growth: a model based on data for North Sea sandeel

“…However, an earlier CPR time-series analysis (Planque & Fromentin 1996) showed a more gradual decline in C. finmarchicus after the mid-1980s rather than the abrupt change suggested by Beaugrand (2004). The more gradual decline suggested by Planque & Fromentin (1996) matches a gradual change from increasing length-atage prior to 1987 to declining length-at-age in the period thereafter off the Scottish coast (Wanless et al 2005, Frederiksen et al 2011, van Deurs et al 2014. Furthermore, in the early 2000s, sandeel recruitment and spawning stock biomass in the North Sea suddenly declined (ICES 2013).…”

Phenological trends and trophic mismatch across multiple levels of a North Sea pelagic food web

“…A major ecosystem regime shift occurred in the late 1980s (Beaugrand 2004), and there have been profound changes in plankton communities (Edwards et al 2002) and fish distributions (Perry et al 2005). Previous studies in this area have investigated phenological changes in species from primary producers to top predators and found contrasting patterns suggestive of trophic mismatch (Edwards & Richardson 2004, Wanless et al 2009, Frederiksen et al 2011. However, none have ad opted an integrated ap proach and compared multiple trophic levels within the same area over the same time period.…”

“…from larval fish captured during CPR surveys and 0-group fish obtained from foraging puffins (see Frederiksen et al 2011 for full details).…”

“…In all cases we report the uncorrected p-values. Phenology data for sandeel hatch date showed evidence of a break-point (Frederiksen et al 2011), and were therefore analysed using a piecewise regression model employing the segmented package in R (Muggeo 2008). In order to assess whether the phenologies of consumers and their prey covaried over time, we examined whether there were pairwise correlations between phenologies of taxa across successive trophic levels.…”

“…Although trophic mismatch theory is most often applied to the timing of peak prey abundance, prey size is a key component and is known to be important for seabirds in our study area . We therefore compared modelled annual length-at-date of 0-group sandeels (Frederiksen et al 2011; sandeel hatched in the current year) with the timing of peak energy demand in each seabird species, which we assumed corresponded to the midchick-rearing period (Drent & Daan 1980) (Fig. 1b).…”