Validating a biophysical dispersal model with the early life-history traits of common sole (Solea solea L.)

Abstract

Larval dispersal and juvenile survival are crucial in determining variation in recruitment, stock size and adult distribution of commercially important fish. This study investigates the dispersal of early-life stages of common sole (Solea solea L.) in the southern North Sea, both empirically and through modeling. Age at different life-history events of juvenile flatfish sampled along the coasts of Belgium, the Netherlands and the United Kingdom in 2013, 2014 and 2016, was determined through the counting of daily growth rings in the otoliths. Juveniles captured between August and October were estimated to be on average 140 days old with an average pelagic larval duration of 34 days. The hatching period was estimated between early April and mid-May followed by arrival and settlement in the nurseries between May and mid-June. Growth rates were higher off the Belgian coast than in the other nursery areas, especially in 2013, possibly due to a post-settlement differentiation. Empirical pelagic larval duration and settlement distributions were compared with the Larvae&Co larval dispersal model, which combines local hydrodynamics in the North Sea with sole larval behavior. Yearly predicted and observed settlement matched partially, but the model estimated a longer pelagic phase. The observations fitted even better with the modelled average (1995-2015) distribution curves. Aberrant results for the small juvenile sole sampled along the UK coast in March 2016, led to the hypothesis of a winter disruption in the deposition of daily growth rings, potentially related to starvation and lower food availability. The similarities between measured and modelled distribution curves cross-validated both types of estimations and accredited daily ageing of juveniles as a useful method to calibrate biophysical models and to understand early-life history of fish, both important tools in support of efficient fisheries management strategies.

Fig 1. Distribution of the eight sampling locations along the coasts of Belgium, the Netherlands (Eems-Dollard estuary), and United Kingdom (Suffolk coast) within the southern North Sea, ICES division 4.c.

Samples (N = 154) were mainly collected at the end of summer in 2013 and 2014, except for the UK samples collected in March 2016. Empty circles indicate the closest buoys to the sampled locations that were used to retrieve coastal SST data. Map modified from www.marineregions.org.

Fig 2

Otolith images of juvenile common sole obtained at 200X magnification used for ageing through the Smartdots software: A) UK; B, C) Belgian and D) Dutch examples. The images are the result of stacking multiple images taken through an Olympus BX53 microscope after subsequent polishing steps to allow for a good visualization throughout the otolith. The white arrow points to the start of metamorphosis, i.e. the end of the larval phase and the initiation of the metamorphosis phase; the blue arrow points to the end of the metamorphosis coinciding with the settlement event. The number of increments between the hatch check and the end of metamorphosis (blue arrow) represents the entire pelagic larval duration (PLD).

Fig 3. Geographic implementation of the Larvae&Co model.

(a) Distribution of the main spawning grounds of Solea solea in the wider North Sea (European Economic Communities delineated by black lines) with contour plots of the mean daily egg production (redrawn from ICES-FishMap [38]. Eggs are released at six known spawning grounds: Off the Belgian coast (BC), off Texel (Tx), the inner German Bight (GB), the eastern English Channel off the French coast (EC), off the mouth of the Thames River (Th) and on the Norfolk Banks (N). (b) Six nursery grounds located in shallow muddy to sandy coastal areas <20 m depth, subdivided according to national boundaries (FR, BE, NL, GE) except for the United Kingdom, where a southern (Tha) and northern (No) nursery area were distinguished. Each colour represents the spatial extent of each nursery. The dotted black lines represent the national Exclusive Economic Zones of the different countries. Adapted maps redrawn with permission from Lacroix et al. [13].

Fig 4. Boxplot of PLD excluded of the yolk sac larval phase predicted for Belgium (BE), the Netherlands (NL) and the United Kingdom (UK) for the period 1995 to 2015.

Triangles highlight the mean values predicted for the years of 2013 (▲) and 2014 (△) in Belgium, 2014 in the Netherlands (△) and 2015 in the UK (△). The observed values are indicated as a square (□) for the year of 2013 in Belgium and asterisks (*) for the year of 2014 in Belgium, 2014 in the Netherlands and 2015 in the UK.

Fig 5

A, B, C: Frequency distribution of the observed (solid line) arrival dates of larval soles at the Belgian and Dutch nurseries in the years 2013 and 2014 compared to the corresponding modelled distribution (dashed line) by the Larvae&Co dispersal model. The y axes represent the relative percentage of juveniles arriving weekly. Predicted arrivals were divided over suggested contributions of potential spawning grounds, i.e. the Belgian coast (BC), off Texel (Tx), the Eastern English Channel (EC) and the Thames estuary (Th), for the different years and locations. D, E: Observed arrival distributions (solid line) compared to the corresponding average modelled arrival distributions (dashed line) over the total (adapted) period (1995–2015) presented in the Larvae&Co model.