Inter-annual variations in spawning and hatch dates of sandeels off Stonehaven .

1. Inter-annual variations in spawning and hatch dates of sandeels off Stonehaven. Alan MacDonald1, Prof. Mike Heath1, Dr Dougie Speirs1 and Dr Simon Greenstreet2, 1Department of Mathematics & Statistics, University of Strathclyde, Glasgow, UK, alan.macdonald@strath.ac.uk 2Marine Scotland Science, Marine Laboratory, Aberdeen, UK Motivation • In this study we examined two hypotheses about variability in hatch timing - • Hatch date is determined by an environmentally cued spawning date and temperature dependent incubation period. • Once a sufficient incubation period has been achieved thenhatching is environmentally cued. • The lesser sandeel (Ammodytesmarinus) is a vitally important source of food for many seabirds and marine mammals, and also subject to commercial fisheries. • The timing (in May/June) of appearance of young-of-the-year recruits (~5 cm length) on the sand-banks off the east of Scotland is critical for the breeding success of several seabirds species (Wanless et al., 2004). • Inter-annual variations in this recruitment timing may be due to variability in the dispersal, hatch-dates and/or growth rates of larvae. 1/3

2. Mean abundance at hatch date for young larvae (3-7 days old when caught). Larvae were sampled at Stonehaven every week between 2000 and 2009. A sample of 7145 were measured to nearest milimetre. Abundance at age Abundance at length Stonehaven HATCH DATE ANALYSIS • Mean hatch date for larvae was found to be in March for each year. • Many larvae do hatch in February and April though. • There is considerable variation in hatch dates throughout the study period. length age Probability of age given length • To calculate hatch dates we had to carry out the following procedure : • The sampled fish had to be aged but it would be very inconvenient to perform otolith analysis on every one! • The solution was to age a small number of sandeels and determine the relationship between length and age. • Now, if we know the length of a sandeel we can determine the probability that it is a certain age. Catch date Catch date This is an otolith, a calcium carbonate structure located behind the brain of bony fish. Daily rings form on the otolith so that a fish can be aged! Length (mm) We used Otolith data from 2000 and 2001 to produce probabilities of length given age. We then used these probabilities to transform measured abundance at length data into abundance at age. We have abundance at age and catch date so hatch dates are calculated as : HATCH DATE = CATCH DATE - AGE 2/3

3. Sandeel eggs undergo a temperature-dependent incubation period. • To estimate spawning dates we need to backtrack from hatch dates by using the relationship between egg development time and temperature. • The average spawning date was day 20 (s.d 4.5 d), varying between day 9 in 2009, and day 24 in 2001 and 2008. • Results were consistent with the hypothesis of a single synchronous spawning event each year with incubation periods of 40 – 50 days. SPAWNING DATE ANALYSIS FUNDING Many thanks to Marine Scotland and MASTS for funding this project. • Mean hatch date varied between day 71 in 2009 and day 87 in 2001 (average of mean hatch date = day 80, s.d 4.7 d). • Mean hatch date + 1 s.d. (hatch end) was significantly correlated to the date of the annual minimum temperature which suggests that hatching could be environmentally cued once a suitable incubation period is reached. • Data for A. marinus incubation times is scarce so we used data on the egg incubation time of a similar species, A. americanus, to estimate spawning dates. Stochastic egg development model for Ammodytesamericanus (a close relative of Ammodytesmarinus) based on Smigielski et al. (1984). (Digitized from Christensen et al. (2008)) Hatch dates occur around the time of the annual minimum temperature Ammodytesamericanus 3/3