Limits of using the commercial catch data
The use of commercial catch declaration limits the possibility to
analyze phenology relative to smaller size classes. In our approach, we
filtered the mature component of the populations based on the size
distribution of each commercial category as described in Alglave et al.
(2023). A similar approach was used in Azevedo and Silva (2020) (though
their approach was more refined as proportion by length was considered
to vary by size category and by zone also) and allowed to map different
age classes of horse mackerel. But a strong limit of our approach is
that it is hardly applicable to smaller size classes. Indeed, commercial
catch declaration data likely provide a biased picture of the
spatio-temporal distribution of smaller fish size class. The minimum
landing size is often pretty high and individuals below this size are
either rejected or not declared (Lehuta and Vermard, 2023). Furthermore,
VMS data are not available for vessels below 12 m that may represent an
important part of coastal vessels fishing near juvenile habitats.
As a consequence, mapping juveniles’ habitat is not possible with catch
declaration data only. In this case, nursery surveys are still the only
available data to investigate juvenile spatio-temporal dynamics and
should be considered as reference (e.g. Nurse, Nourseine et
Noursom - Delaunay and Brind’Amour, 2018).
Also, another drawback when using commercial data is the targeting
behavior of fishermen. This can lead to biased spatial predictions and
to overestimated estimates of biomass. Here, we neglected it as trawlers
don’t have a strong targeting behavior. Still, the framework developed
in Alglave et al. (2022) could be useful in the case of stronger
preferential sampling (see Quemper, 2021).