The usefulness of a multidisciplinary approach to investigate the differentiation of Atlantic swordfish (Xiphias gladius L. 1758)
Author(s)
De Carvalho dos Santos Garcia Guerreiro, Alexandra
Date Issued
February 10, 2011
Type
Doctoral Thesis
Abstract
The swordfish (Xiphias gladius L. 1758) is a highly migratory species that is
harvested over its entire distribution range. For management purposes, the International
Commission for the Conservation of Atlantic Tunas (ICCAT) established two distinct
stocks within the Atlantic, the North and South, separated at 5ºN. The existing stock
boundaries were established at a time when biological evidence for stock structure was
conflicting and inconclusive and were not intended to represent discrete populations.
Thus, the actual boundary was arbitrarily placed on 5ºN for the practical purpose of
alignment with ICCAT’s existing statistical reporting areas. Taking into consideration
the uncertainties concerning the Atlantic swordfish stock limits, as well as the
implications that such doubts have for management purposes, the present work aimed to
evaluate the accuracy of the location of the current boundary between the North and
South Atlantic stocks and assess the degree of subdivision and discreteness between
them. In order to achieve such goals, a multidisciplinary (holistic) approach was carried
out which incorporated a set of different aspects of the swordfish biology (parasites;
genetic, morphometry and age and growth). The study of parasites allowed to gather
new information on the relationship between infection levels and host size and sex and
gave indications about the possible use of some taxa as “biological tags” for stock
discrimination. The relationship between metazoan parasites and host sex did not
revealed statistically significant correlation in Atlantic tropical-equatorial area. Infection
levels with T. integrum and T. coccineum were observed to be significantly negatively
correlated to host size. Infection levels by endoparasites observed in swordfish from
Atlantic tropical-equatorial area may be related to host feeding behaviour and the
availability of the parasites’ intermediate and final hosts in the area studied.
Parasitological data acquired from tropical-equatorial area, northwest, central north, east
tropical and central south revealed the association of some parasite taxa with distinct
areas based on their presence/absence and infection levels. Hysterothylacium
corrugatum (s.l.) and Rhadinorhynchus pristis were mostly associated with swordfish
caught in the northwest, and Anisakis paggiae and H. incurvum with fish caught in the
southern areas (east tropical, tropical-equatorial and central south). Anisakis simplex s.s.
could be used as a marker for all northern samples. Parasitological data on Atlantic
swordfish seems to support the existence of at least two discrete stocks in this water
basin. The genetic diversity within and between the studied areas was also investigated
at the DNA mitochondrial control region (mtDNA D-loop). The analysis of molecular
variance (AMOVA) showed that genetic variation was mainly due to differences within
rather than between the studied areas. Additionally, the phylogenetic analysis did not
show evident relationships among haplotypes from all areas, as they are randomly
allocated across the tree diagram. However low, but significant FST values were
recorded when comparing equatorial samples with all the other from North Atlantic.
Morphologic differentiation of swordfish caught in east tropical, equatorial and south
central Atlantic waters was investigated using morphometric characters of the second
ray of the anal fin. Regression analysis revealed that fin ray growth in width was less
variable than growth in height and fin ray width increases more rapidly than fish length.
Discriminant analysis suggested significant phenotypic heterogeneity among areas. The
tropical samples were the most isolated, while the equatorial and south central samples
were clustered together. The age and growth of the swordfish were estimated using the
anal fin spines of swordfishes caught in eastern Atlantic waters. The samples consisted
of 12 age-classes, with age-classes 2 and 3 being the most represented. Marginal
increment analysis showed that growth bands 1 to 7 were deposited annually. The
monthly mean MIR estimated and the month effect on MIR provided by the GLM
suggested that an annulus is formed in middle spring. The swordfish length-at-age was
back calculated using two methods and estimated by the standard and generalised von
Bertalaffy models. Overall, growth rates were higher during the first year of life and
slowed appreciably afterwards for both sexes. However, females still growing faster
than males and consequently reach larger sizes than males. The standard von Bertalaffy
function provided a more realistic description of age-size relationship of swordfish over
the age ranges considered. Growth parameters for the pooled sexes were described as:
L∞=332.104, K=0.077; t0=-2.778. Finally, the overall information gathered by the
different techniques used seems to support the existence of different stocks of swordfish
in the Atlantic Ocean; they also revealed that migratory movements between the
northern and southern areas are occurring and that mixing occurring especially in the
equatorial area. However, although no precise boundaries could be established for
Atlantic swordfish stocks, the current data does not support the need to change the
current fishery management boundary, placed at 5ºN.
Additional information
Dottorato di ricerca in Ecologia e gestione delle risorse biologiche
File(s)![Thumbnail Image]()
Name
agarcia_tesid.pdf
Size
1.65 MB
Format
Adobe PDF
Checksum (MD5)
74851a70ad3a699b8e1a5d9baa362e10