4.3 Correlation between relative specialization and prey species
composition
Our data suggest that the higher proportion of benthic species consumed,
the relatively more specialized the diet of the predator. This pattern
was observed in the full dataset, as well as when female and male data
were considered separately (Tables 2-5). This information ties to our
knowledge of the foraging patterns of male versus female harbor seals in
the region. Females more often perform deeper foraging dives (Wilson et
al., 2014) and eat more benthic species than males, who eat more pelagic
species (Schwarz et al., 2018). In Scotland, harbor seal scat samples
represented either a largely pelagic foraging strategy or largely
benthic foraging strategy (Tollit, Greenstreet, & Thompson, 1997), and
males had larger range and duration in foraging trips (Thompson, Mackay,
Tollit, Enderby, & Hammond, 1998), suggesting that the separation
between the two foraging strategies is not just a regional phenomenon.
The consistency across sexes indicates this pattern is reflective of
foraging strategies specific to the ecology of prey species, and not
just indicative of different diet preferences between males and females.
We hypothesize that this pattern was caused by higher variability in
benthic environments (Lalli & Parsons, 1997). If prey have more
variable life strategies, a single foraging strategy will not suffice to
catch them all. Because an organism is likely limited in the number of
foraging strategies at which it can be effective, an individual could be
limited in the number of prey species it can exploit.
There is the possibility that the sex of the individual determines the
level of specialization regardless of the prey consumed. However, the
consistency of benthic prey being associated with a relatively
specialist diet, and pelagic prey being associated with a relatively
generalist diet, in the complete, only female, and only male datasets
suggests that the ecology of the prey species and the sex of the seal
was driving the observed pattern. This idea is supported in other
literature as well. Individual male harbor seals in Nova Scotia use
different behaviors when foraging for benthic versus pelagic prey
(Bowen, Tully, Boness, Bulheier, & Marshall, 2002) and larger seals are
more likely to forage in pelagic environments regardless of sex
(Bjorkland et al., 2015).
If prey species ecology is driving specialization levels, it is
especially interesting to consider harbor seal consumption of juvenile
Salmoniformes. For example, juvenile sockeye (Oncorhynchus nerka ,
Walbaum 1792) correlated with a generalist diet (rho = 0.22, p = 0.004).
This could indicate that seals were not seeking out juvenile
Salmoniformes specifically but rather eating them as a byproduct of
focusing on fish that match the image of forage fish (e.g. small and
silver) while conducting pelagic foraging strategies. This is just one
example of how understanding the level of specialization could deepen
our scope of knowledge regarding harbor seal impacts on prey species of
concern.
There are a few notable limitations to this study. First, there was the
potential for variation in sample size to introduce bias. However, there
were no discernable patterns between sample size and average relative
specialization by season (Figures 2, 3). We also included sample size as
a random factor in the model to account for any bias introduced there.
Hence, any bias introduced by sample size was likely minimal. Second,
because scat were collected from the same haul-out multiple times there
is a chance that some scat collected came from the same individual.
However, this chance is low as Rothstein et al. (2017) estimated the
sampling scheme to track five individuals at Cowichan Bay (i.e. a single
haul-out) as 440 samples over 22 sampling bouts. Compared to the 1,083
samples used in this analysis from five different haul-outs, it seems
unlikely there was a high rate of resampling the same individuals.
Third, there are biases in the metabarcoding PCR process for determining
diet (Thomas, Jarman, Haman, Trites, & Deagle, 2014). The prey
proportions recorded for each sample are not directly proportional to
the amount of prey that was ingested (Bowen & Iverson, 2012; Thomas et
al., 2014). However, this approach is accepted to be semi-quantitative,
biases are assumed to be consistent between samples (Thomas et al.,
2014), and the approach has been successfully used in other studies
(Deagle et al., 2010; Pompanon et al., 2012; Schwarz et al., 2018;
Thomas et al., 2014; Thomas et al., 2017). Furthermore, this approach is
superior to the alternative occurrence-based methods for generating diet
proportions (Deagle et al., 2019). On a related note, these molecular
methods do not provide data that directly equate to counts of prey
consumed. But, if individuals within a local group encounter the same
size distribution of a given prey species, then diet proportions
represent the same relative relationship of prey capture decisions.
Further investigation into potential biases introduced by using
proportion type data would be useful as this methodology has many
benefits and is a valuable molecular technology that should be applied
in the future.