Alan Couch, Fiona Dyer, Mark Lintermans , and Patrick Ross-Magee

1. Alan Couch, Fiona Dyer, Mark Lintermans, and Patrick Ross-Magee Institute for Applied Ecology, University of Canberra Performance of Small Mesh Drift Nets in Rivers. Background: Deploying small-mesh drift nets in rivers is a well-established method for sampling drifting fish larvae and eggs. Quantitative comparisons are sometimes made on the basis of numbers of larvae captured per unit volume or time.1,2,3 But is net performance decay a source of error? Method: A GoPro™ camera was mounted inside a drift net to record the change in flow rate over soak time (1 to 5 minute intervals for 3 hours). The angle (Ø) of the General Oceanics™ flow meter relative to the net mouth was measured at the same intervals. Net mouth Larva? (Photo - SA Govhttp://www.sardi.sa.gov.au/) • Flow Results: • Although a small number of net nights were sampled (7 nights at 3 locations), variance in the change in flow within and between sites was observed – even during soak times as little as 2 hours. In one case (L2) there was almost no change in flow over 180 minutes but at the most extreme (B1), the flow dropped from 8.9 m3/min to 1.5 m3/min in just 160 minutes. • Critical Swim Speed? • Where does your target species critical swim speed fit on the y axes? In these nets: • 5 m3 min-1 is about 44 cm s-1 • 2 m3 min-1 is about 18 cm s-1. • This is well below the critical swim speed of many Australian freshwater fish larvae (Kopf et al, 2014). • Turbulence Results: • In some cases deviation from Ø = 0 becomes more erratic over time indicating turbulence or, even eddies so that although flow is recorded it may be lateral to, or even opposite to, the direction expected. The photos below show flow meter for net event N1 before (left 4 frames) and after (right 4 frames) turbulence started at 1.5 hours after set. • Conclusion: If volumetric or temporal estimates are made solely from total throughput, they may sometimes be misleading and at worst make comparisons meaningless. Dedicated data logging flow meters exist but are prohibitively expensive for routine sampling. Researchers may find that underwater cameras provide a cost effective alternative to understand net performance decay for their application. Flow Direction Ø References: Gilligan, D., & Schiller, C. (2003). Downstream transport of larval and juvenile fish in the Murray River NRMS Project No. R7019, July 2003, (50). Humphries, P. (2005). Spawning time and early life history of Murray cod, Maccullochella peelii (Mitchell) in an Australian river. Environmental Biology of Fishes, 72(4), 393–407. Koehn, J.D. & Harrington, D. J. (2006). Environmental conditions and timing for the spawning of Murray cod (Maccullochella peelii) and the endangered trout cod (M. macquariensis) in southeastern Australian rivers. River Research and Applications 22(3),327–342 Kopf, S. M., Humphries, P., & Watts, R. J. (2014). Ontogeny of critical and prolonged swimming performance for the larvae of six Australian freshwater fish species. Journal of Fish Biology, 1–22. Faulkner, H., & Copp, G. H. (2001). A model for accurate drift estimation in streams. Freshwater Biology, 46, 723–733. Tonkin, Z., King, A., & Mahoney, J. (2007). Management and Ecological Note Testing a modification to a standard passive drift net to capture drifting ichthyofauna. Fisheries Management and Ecology, 14, 299–301.