Overexploitation and decline in kelp forests inflate the bioenergetic costs of fisheries.

Aim: Fisheries ecosystems are subject to long‐term shifts in food web structure as a result of exploitation and environmental change. These shifts can be gradual and unresolved by decadal‐scale time series. The aim of our study was to determine long‐term legacy effects of overexploitation and changes to the composition of basal organic matter sources on the trophic structure of marine food webs. Location: New Zealand. Time period: Approximately 1250 AD to the present; the complete history of human occupation in New Zealand. Major taxa studied: Mesopredatory fishes. Methods: We used whole tissue (δ13C and δ15N) and amino acid‐specific (δ15NAA) stable isotope analyses of bone collagen and muscle tissue from five fishes spanning the period of human occupation of New Zealand to resolve the bioenergetic consequences of long‐term shifts in food web structure. Stable isotope analysis of amino acids provided the basis for resolution of changes in trophic level in the absence of information on δ15N at the base of the food web during pre‐industrial times. Results: Our analyses indicate likely declines in the contribution of organic matter derived from kelps in four species, and intraspecific increases in trophic levels in three species of the five fishes studied between European colonization (AD 1650–1800) and the present‐day regime of industrialized fishing and environmental change (AD 1953–2018), but little change during the prehistoric time period spanning early Māori occupation (AD 1250–1450) to European colonization. Analysis of the bioenergetic costs of the observed shifts in food web structure indicate a 179% increase in basal organic matter requirements to support mesopredatory fish. Main conclusions: These data provide a rare case study on the consequences of legacy effects of exploitation and environmental change for bioenergetics of fish communities relevant to ongoing changes in global marine ecosystems. Overexploitation and decline in kelp forests have inflated the bioenergetic costs of these fisheries. [ABSTRACT FROM AUTHOR]