Mortality rates of early developmental stages of freshwater drum and sunfish in the Upper Mississippi River System

Larval fish were sampled with plankton nets at two backwater and
two main channel stations in Pool 8 in 1990, and at one main
channel and one backwater station in Pools 8 and 14 in 1989 (four
stations each year).  Growth of sunfish (Lepomis spp.) and
freshwater drum (Aplodinotus grunniens) was estimated with a
length-based method and an otolith-based method.  For each method,
instantaneous mortality was estimated by regressing the natural
logarithm of fish catch for each 1-mm size group against the
group's estimated age.  For freshwater drum, length-based and
otolith-based estimates of mortality were similar at three of four
main channel stations.  However, for larval sunfish, the
otolith-based method provided more precise estimates of growth
than the length-based method at two of three stations.  Estimates
of mortality were similar for both methods in Pool 14, where
catches were higher, but the length-based method gave
significantly higher estimates in Pool 8, where the catches were
lower.  Both length-based and otolith-based methods can
effectively estimate larval fish growth and mortality, but each
may be more appropriate for certain situations.  Length-based
methods require large catches to produce reliable estimates, but
are less labor intensive than the otolith-based method.