FRDC 2021-020: Stable isotopes: a rapid method to determine lobster diet and trace lobster origin?

Scientists at IMAS conducted a stable isotope study on Southern Rock Lobster (Jasus edwardsii) collected from Southern Australian sites to determine whether or not capture site could be determined post-harvest.

Background
The project was undertaken because of interest to industry - will this technology allow rapid post-harvest determination of capture site? Such a method would be of great value to the Southern Rock Lobster industry as it could allow isolation of stock abnormalities and validation of catch data.

Aims
There were two major aims for this project:
1. To assess if stable isotope signatures can be used to trace lobster origin
2. To detail a suitable method which could be used to collect this data in an industry setting, should the results prove valuable
These two aims were directly related to the industry’s interest in the method, these took priority for this project and report and we provide conclusions on these.
Parallel research aiming to use stable isotopes to evaluate lobster diet will be publish as part of the lead author’s PhD program in 2024.

Methodology
The project was split into two parts:
1. A sample set of lobsters taken from 8 sites around Southern Australia were used to assess the viability of using stable isotopes to trace lobster origin post-harvest. Each lobster’s isotope signature was measured and then assigned a “most-likely” site of origin based on the signatures of others in that site. We then looked at how accurate our assignment process was.
2. Lobsters from the wild and tank experiments were sampled to determine whether there was any difference in isotopic signature between different tissues within the same individual (i.e.does the leg muscle vary from the dorsal muscle or from the shell?). Wild lobsters were also sampled across different size classes and depths to assess whether these factors influence mean isotopic signature within a capture site.

Key Findings
Our key findings are:
• δ13C and δ15N isotopes from lobsters can be used to gain information on the origin of catch but not on a fine geographic scale. Accuracy of origin assignment is heavily dependent on site specific reference data and prior known information. For example, δ13C and δ15N isotopes can be helpful in validating (yes/no) the possibility of a lobster originating from a labelled origin but would be limited in determining a location without any prior reference.
• We show the use of leg tissue for isotope analysis in lobsters is highly correlated with dorsal tissue, allowing for non-lethal/non-destructive sampling in future
• We show that depth does not necessarily influence isotope signature within a fishing site (this requires more site-specific sampling)
• We show that in some sites, size class of lobster can influence isotopic signature - this is not the case at all sites, but we recommend sampling within size classes for reference datasets
• We found that isotope assimilation time is within months, therefore recommend similar sampling time (i.e. same season) for reference and test samples

Implications for relevant stakeholders
This research is relevant to multiple stakeholders:

INDUSTRY & MANAGERS
We found that although there is some differentiation between lobster isotope signatures of different capture sites, stable isotope analysis is unlikely to be able to trace exact origin of lobsters post-harvest. Due to the wide variation in the isotopes and lots of overlap between sites, isotope signature alone will not categorically assign lobsters to capture site with a high level of accuracy. Depending on the geographical scale required, isotope signatures can be accurate in assigning to larger regions (such as state, rather than sites within a state), but this still depends on the regions to be distinguished.

SCIENTISTS & PRACTITIONERS
In future, we recommend the use of non-lethal sampling for stable isotope analysis of Southern Rock Lobsters. Isotope variation can occur between tissues within individuals, between sites, seasons and depths so we recommend designing isotope studies accordingly.

Recommendations
We recommend non-lethal samples, such as a lobster leg rather than a dorsal muscle, can be used for isotope analysis. For tracing purposes and from the results we show here we recommend using size- and season-specific reference datasets but not necessarily depth-specific.
Larger sample sizes for reference isotope datasets would improve our understanding of the variance in signature associated with each site. Similarly, sampling lobsters at much deeper depths (i.e. 100m) will help to inform sampling protocols.