Age, growth, and mortality of Joffrichthys triangulpterus (Teleostei: Osteoglossidae) from the Paleocene Sentinel Butte Formation, North Dakota, U.S.A.

ABSTRACT

Fossilized Joffrichthys triangulpterus scales collected from an ancient lake site in the Paleocene Sentinel Butte Formation near Almont, North Dakota, provide an opportunity to estimate growth, natural mortality, and survival for this species. Based on 166 J. triangulpterus scales, fish ages ranged from zero through nine years old. Four scale-covered skeletons were also recovered and aged at zero, zero, one, and four years of life. Based on an age-frequency histogram, age groups zero, one, and two are clearly under-represented at the Almont site, suggesting they likely inhabited different areas of the lake system, perhaps due to habitat partitioning between juveniles and adults in the lake. Instantaneous natural mortality (M) determined from age-frequency data indicates the average annual mortality of age three and older J. triangulpterus was −0.456 (r ² = 0.958), which is similar to numerous extant fish mortality curves. A Von Bertalanffy growth curve (r ² = 0.977) indicates that growth of J. triangulpterus was rapid in the first three years of life, attaining over 70% of their adult size before reaching an asymptote in growth at age nine. The maximum body size attained by one individual of J. triangulpterus was estimated at 316 mm total length. The growth pattern and mortality rates exhibited in J. triangulpterus are generally similar to those of numerous extant fish taxa found in temperate latitudes.