A Resolution to the Blue Whiting (Micromesistius poutassou) Population Paradox?

Abstract

We provide the strongest evidence to date supporting the existence of two independent blue whiting (Micromesistius poutassou (Risso, 1827)) populations in the North Atlantic. In spite of extensive data collected in conjunction with the fishery, the population structure of blue whiting is poorly understood. On one hand, genetic, morphometric, otolith and drift modelling studies point towards the existence of two populations, but, on the other hand, observations of adult distributions point towards a single population. A paradox therefore arises in attempting to reconcile these two sets of information. Here we analyse 1100 observations of blue whiting larvae from the Continuous Plankton Recorder (CPR) from 1948–2005 using modern statistical techniques. We show a clear spatial separation between a northern spawning area, in the Rockall Trough, and a southern one, off the Porcupine Seabight. We further show a difference in the timing of spawning between these sites of at least a month, and meaningful differences in interannual variability. The results therefore support the two-population hypothesis. Furthermore, we resolve the paradox by showing that the acoustic observations cited in support of the single-population model are not capable of resolving both populations, as they occur too late in the year and do not extend sufficiently far south to cover the southern population: the confusion is the result of a simple observational artefact. We conclude that blue whiting in the North Atlantic comprises two populations.

Oceanographic variability shapes the spawning distribution of blue whiting (Micromesistius poutassou)

“…For the purposes of this analysis and following previous work (Pointin & Payne, ), the CPR data was geographically restricted to the region where blue whiting larvae were most commonly observed (Figure a), and to the period between February and June, covering more than 99% of all larval presences (Figure b; Pointin & Payne, ). The CPR data consisted of 34,422 CPR observations over 54 years (1951–2005) including 1,122 presences of blue whiting larvae between February to June (Figure b).…”

“…It is hypothesised that blue whiting larvae perform diel vertical migrations by ascending to the surface waters during dusk and dawn (Hillgruber & Kloppmann, ), which could affect the CPR's capture efficiency as indicated by Pointin and Payne (). To quantify light conditions in the upper water column at the time of capture, the angle of the sun measured from the horizon upwards at the time and point of observation was calculated using the solarpos() function in the maptools package (version 0.8.39; Bivand & Lewin‐Koh, ) in R.…”

“…This species migrates annually between its feeding grounds in the Norwegian Sea to its spawning region west of Great Britain and Ireland (Figure ; Bailey, ; ICES, ,b). While there are indications of limited spawning activity outside of the main spawning area (Bailey, ; Bainbridge & Cooper, ), most spawning takes place from March to April along the European Continental Shelf edge and over banks to the west of Great Britain and Ireland (Bailey, ; Pointin & Payne, ).…”

“…Further CPR data revealed a great abundance and broad spatial distribution of blue whiting larvae in the North‐East Atlantic, causing Raitt () to conclude that the spawning stock would be sufficiently large for commercial exploitation and resulting in fisheries biologists becoming interested in the species (Bainbridge & Cooper, ; Polonsky, ). Since the subsequent development of the fishery in the late 1970s, CPR larval data has also been used to resolve the population structure of blue whiting (Pointin & Payne, ). In addition, Hátún, Payne & Jacobsen () used a spatial subset of CPR data from 1951 to 1970 along a narrow axis across the Rockall Trough to support their hypothesised link between the marine environment and blue whiting spawning.…”

The impact of environmental variability on Atlantic mackerel Scomber scombrus larval abundance to the west of the British Isles

“…As well as zooplankton and phytoplankton (the main focus of the CPR), fish larvae have been analysed up until-through 2005 (Edwards et al, 2011). The value of the CPR fish larvae dataset, with its extensive spatio-temporal coverage, has been recently demonstrated with studies on longterm changes over decadal scales in the abundance and distribution of fish larvae in relation to physical and biological factors (Pitois et al, 2012), on blue-whiting populations in the North Atlantic (Pointin and Payne, 2014), and on the use of a larval index to inform stock assessments for sandeels (Lynam et al, 2013) and mackerel (Jansen et al, 2012b) in the North Sea. Analyses in the North Sea (Pitois et al, 2012) suggest that the larvae of clupeids, sandeels, dab and gadoids seem to be affected mainly by changes in the plankton ecosystem via bottom-up effects, while the larvae of migratory species such as mackerel respond more to hydrographic influences.…”