How are you searching? You may want to use PUBMED and AGRICOLA. You would find assays for different digestive enzymes in the series, "Methods in Enzymology", a long-standing series.  http://www.ncbi.nlm.nih.gov/pubmed

THE CONCENTRATIONS OF PROTEASES, AMYLASE, AND LIPASE IN CERTAIN MARINE FISHES. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.314.6521&rep=rep1&type=pdf

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2762538/  Digestive enzyme activities and gastrointestinal fermentation in wood-eating catfishes

http://www.ncbi.nlm.nih.gov/portal/utils/pageresolver.fcgi?recordid=5619ee7835fd1e11e9c52d5a  Effects of Replacement of Fish Meal by Soy Protein Isolate on the Growth, Digestive Enzyme Activity and Serum Biochemical Parameters for Juvenile Amur Sturgeon (Acipenser schrenckii).

Here is the protocol that i used. Gupta et al. 2015

For spectrophotometric analysis various popular mehtods are available

Lipase: Winkler and stuckmann 1979

Proteases Casien hydrolysis assay

Good luck

doi:10.1016/j.cbpb.2006.09.00

see this article.