More but smaller: Marine heatwaves exacerbate size truncation in overfished fish communities in the Skagerrak

As we are progressing through the Anthropocene, it is imperative to
understand how ecosystems degraded by historical impacts respond to future
pressures. Particular attention should be directed at changes in the
distribution and function of foundation species due to their
disproportionately important ecological roles. Forest-forming kelps are in
decline and are being replaced by turf-forming algae in coastal areas
globally, as a consequence of increasingly severe marine heatwaves (MHW).
This replacement represents a miniaturization of the three-dimensional
habitat and a change in habitat functions for many associated species.
Ecologically, economically, and culturally important coastal fish species
depend on kelp forests in different life stages from egg to adult and may
be affected in various ways. Many of these species already suffer from
centuries of overexploitation, severely reducing abundances, truncating
sizes, and changing age distributions. Here, we studied an overfished
coastal ecosystem and assessed the impact of the transition from kelp to
turf on the structure and function of associated fish communities.
Specifically, we analyzed differences in abundance, body size, and
composition of benthic fish at sites with variable cover of kelp and turf.
We found higher abundances of small fish with increasing cover of turf
relative to kelp. We also found smaller fish within the common families of
gobids and gadids. Thus, MHW exacerbate size truncation and trophic
downgrading in coastal fish communities. However, no differences in
abundances of fish in the water column were detected during the night, and
the average size of fish at turf-dominated sites was even larger. We
interpreted this as roving predatory fish during night-time being less
affected by the shift in habitat and smaller-bodied prey avoiding the
water column in open turf habitats.