To state what is likely already obvious to many on here: I would expect that adult wild fish - given the opportunity - can and do dive deeper below any surface plankton bloom of harmful algal blooms such as
Heterosigma spp., and/or can swim out of a localized area with a heavy bloom.
Juveniles - on the other hand - live in the shallows and don't have the same abilities as adult fish to dive deep; and I would also likely expect to see an increase in predators on juvies in deeper waters.
So, it is likely that any
Heterosigma spp. bloom in the spring that co-occurs in time/space with juvie outmigration would have some impact:
https://www.sciencedirect.com/science/article/pii/S1568988310000867
Farmed fish are particularly sensitive to these blooms since they can't move, and can only dive as deep as the nets allow.
from:
https://en.wikipedia.org/wiki/Heterosigma_akashiwo
The 1997 H. akashiwo bloom in British Columbia, for example, coincided with a dramatic increase in mortality of captive salmon.[15] H. akashiwo contributed to the loss of over 1,000 tons of Atlantic salmon in 2001.[6] A bloom in Puget Sound in 2006 led to the loss of $2 million of farmed salmon.[7] In 2014, a bloom near Port Hardy, British Columbia, killed nearly 280,000 Atlantic salmon.[16] In 2018, a bloom near British Columbia killed near 250,000 Atlantic salmon at two seafood farms.[17] A 1995 article noted that the global distribution of H. akashiwo is increasing, as is the frequency of H. akashiwo HAB formation.[18]
Looks like this will be getting more common with global warming:
https://www.sciencedirect.com/science/article/pii/S1568988308001273?via=ihub
www.pnas.org/cgi/doi/10.1073/pnas.1619575114
http://dx.doi.org/10.1016/j.foodres.2010.02.010