What are the ages of yabbies in dams?
Crayfish can’t be aged like fish because they don’t have any permanent hard parts (scales, otoliths, spines, bones) to form growth rings. External tags attached to the shell are lost at ecdysis; an external tag planted in the flesh at the join between cephlothorax and abdomen, and not lost at ecdysis, is used for large rock lobsters but is not successful for small crayfish. Crayfish have to be killed to retrieve magnetic internal tags or to use a new microscopical chemical ageing method on brains. We follow growth of yabbies in our study dam, and estimate their total numbers, by punching marks on their uropods and pleura which correspond to weight grade category and month of capture, resp.. These marks last through several ecdyses. At Christmas, you’ll need a mosquito net to catch the tiny, newly released young-of-the-year from the spring spawning. Most of the yabbies caught in traps then are from the two spawnings in the previous year (one year olds) and they range from 10 grams (tail
