Importance of drainage-specific growth curves for back-calculating spawning periodicity of federally endangered Colorado Pikeminnow and Razorback Sucker

Authors: Martinique Chavez, Stephani Barkalow, Steven Platania

Year: 2019

Abstract

San Juan River larval fish surveys have occurred annually since 1998 to monitor reproduction by federally endangered Colorado Pikeminnow, Ptychocheilus lucius, and Razorback Sucker, Xyrauchen texanus. As part of this monitoring effort, spawning periodicity of both species was back-calculated annually using growth equations developed in the Upper Colorado River basin. While back-calculated spawning dates generated from these models were assumed to be approximations, use of the equations was limited, particularly for Razorback Sucker, because older age-0 fishes produced unfeasible spawning dates. In 2017, San Juan River basin-specific growth curves were developed for Colorado Pikeminnow and Razorback Sucker, using larvae collected in the San Juan River from 2009 to 2017. These growth curves were used to estimate age in days from standard length of age-0 fish. Back-calculated age was used to calculate spawning periodicity and was compared against spawning periodicity calculated with the upper Colorado River basin equation and daily otolith ages for both species. Initial results indicated that spawning periodicity produced by San Juan River basin growth curves was significantly different from those produced by the Upper Colorado River basin growth curves but not from spawning dates calculated from otolith ages. We then used species-specific San Juan drainage growth curve equations to back-calculate spawning dates of all larval Colorado Pikeminnows and Razorback Suckers collected in the San Juan River between 2000 and 2018. Spawning periodicity from the San Juan River equations was compared against that of the Colorado basin equations using ANOVA. The San Juan River is largely a dam-regulated system with water release from Navajo Dam managed to mimic a natural flow regime. Inaccurate back-calculation of spawning dates could result in missed opportunities for water release from dams or inadvertently harm larval fishes.