Investigating long-term temporal changes in trophic structure of fish communities: insights from whole muscle tissue and compound specific isotopic analysis of amino acids

Food web structure is both a key driver of ecosystem processes and an important influence on fish populations. Uncovering food web parameters that predate anthropogenic pressures remains a challenge in applied ecology, hampering comparisons with present environmental states. Isotopic analysis is currently emerging as a potentially important tool for reconstructing food web structures from museum specimens. While ecological estimates from whole muscle isotopic analysis depend on primary producers sampling, estimates from specific amino acids (AA) are not affected by changes at the base of the food web.

We applied whole muscle tissue and compound-specific isotopic analysis of amino acids to nine commercially important fish species collected from New Zealand waters between 1953 and 2018. Raw isotopic data from museum specimens were corrected for preservation, fixation, lipid content and variation of atmospheric δ¹³C, and trophic levels (TL) were estimated from both whole tissue and AA values for nine commercially important species of fish.

The relationship between TL estimates from whole muscle tissue and AA varied between species feeding habits and the ¹⁵N enrichment factor used. Results from AA estimates demonstrated that Nemadactylus macropterus, Hoplostethus atlanticus, Pseudophycis bachus and Genypterus blacodes had an increase in TL with time. There was a considerable increase in fisheries landings between 1970 and 2000 for those species, followed by a decrease due to reduction in stock size. Species with stable landings over time showed no shift in TL, except for Thyrsites atun, a nektonic feeder that showed a decrease in TL between 1953 and 2017.

Our findings suggest that combined whole muscle and AA isotopic analysis are a powerful tool for accessing ecological information from past environments, being potentially important to resolve long term ecosystem-scale effects of anthropogenic activities.