Was the stream draining an agricultural landscape?
Do you have measurements of algae in that stream?
I did some stream isotope research in New Brunswick for my graduate degrees.
It was rare, but not uncommon to find algal values near -15 per mil. Some
species of aquatic algae can utilize HCO3- instead of aqueous CO2 during
photosynthesis. This causes there d13C values to be more enriched. So,
depending on the d13C of the DIC in your system, the concentration of DIC,
and the photosynthetic mechanism of the algae, it's not that unlikely to see
-15 per mil in stream fishes if they are completely reliant on the
autochthonous portion of the food chain.
To rule out C4 and terrestrial insects, I would take a look at their gut
contents. Not many bugs prefer eating C4 plants. Unless, of course, you're
in an agricultural landscape (e.g., corn).