82 
Fishery Bulletin 11 6(1) 
4000 
3500 
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c 2000 
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□ Madeira □ Canary Islands ■ Portugal mainland 
Figure 1 
Landings (in metric tons) of blue jack mackerel (Trachurus picturatus) 
caught off Peniche, a city on the coast of mainland Portugal, off the Madeira 
archipelago, and off the Canary Islands during 2000-2016. Source: Institute 
Nacional de Estatistica, Portugal 
Friedland, 1999). Consequently, phenotypic and genetic 
variation among fish within a species can occur as a 
result of isolation, and such variation can be seen as 
a basis for separation of fish into groups that can be 
managed as distinct populations (Turan, 2004). How¬ 
ever, areal differentiation within a species may also 
arise from other factors, such as 1) countergradient 
variation when environmental influences reinforce the 
genetic differences between populations (Yamahira and 
Conover, 2002; Barria et ah, 2014), 2) thermal reaction 
(Yamahira et ah, 2007; Yamahira and Takeshi, 2008), 
and 3) local adaptation and adaptive phenotypic plas¬ 
ticity (Yampolsky et ah, 2014). 
Identification of fish stocks by using shape analysis 
has evolved from measuring simple linear distances to 
deriving geometric variables (Stransky, 2014). The de¬ 
velopment of image processing tools has facilitated the 
change from traditional morphometries to more com¬ 
plex geometric functions (Cadrin and Friedland, 1999) 
and has increased the power of morphometric analy¬ 
sis for population discrimination (Rohlf and Bookstein, 
1990; Marcus et ah, 1996; Cadrin and Friedland, 1999; 
Cadrin et al., 2014). Geometric approaches to morpho¬ 
metric analysis are often classified as either “landmark 
methods” (Cadrin et al., 2014) or “outline methods” 
(Bookstein et al., 1985; Marcus et al., 1996). Landmark 
methods are based on anatomical points or landmarks 
to analyze morphometry, and outline methods are used 
to identify different patterns of otolith shapes. 
Otolith shape is species specific, is less variable 
than body growth for fish (Campana and Casselman, 
1993), and the appearance and shape of the otolith 
often vary geographically for any given species (Tuset 
et al., 2008). Unlike scales and bones, otoliths grow 
during the entire life of a fish, and re-absorption or 
alteration of otolith material once it has been deposited 
is unlikely (Campana and Neilson, 1985; Casselman, 
1987). Consequently, otoliths remain unchanged during 
short-term changes in fish condition (e.g. starvation) 
(Campana and Casselman, 1993). Nevertheless, envi¬ 
ronmental and genotypic factors can induce differences 
in metabolism and growth that might influence otolith 
shape (Cardinale et al., 2004). In addition, the otolith 
shape can vary substantially if genetic or environmen¬ 
tal differences persist and the populations remain par¬ 
tially isolated and inhabit different environments (Bird 
et al., 1986; Campana and Casselman, 1993; Begg and 
Brown, 2000; Begg et al., 2001; Smith et al., 2002). 
The blue jack mackerel, Trachurus picturatus 
(Bowdich, 1825) (Osteichthyes, Carangidae) is an oce¬ 
anic pelagic species ranging to depths of at least 370 
m (Smith-Vaniz, 1986) (575 m in the Azores, Menezes 
et al., 2006) and can be found from the Bay of Biscay 
(France) southward to Morocco and eastward into the 
Mediterranean Sea (Smith-Vaniz, 1986). It is a commer¬ 
cially important species with highly variable landings. 
On the Portuguese mainland, for example, landings 
have doubled over the last decade (INE 1 ), while landings 
off the Canary Islands have tripled (Castro 2 ) (Fig. 1). 
Apart from one very recent study of parasites in 
1 INE (Institute Nacional de Estatistica). 2013. Estatisticas 
da Pesca 2012, 133 p. Instituto Nacional de Estatistica, I.P., 
Lisboa, Portugal. [Available from website.] 
2 Castro, J.J. 2017. Personal commun. Faculty of Marine 
Sciences, University of Las Palmas de Gran Canaria. 
