Research involving charter boat captains provides critical data on reproductive biology.

Research Need                          

The Mid-Atlantic and South Atlantic fishery management councils jointly manage blueline tilefish. In the South Atlantic, recreational fishing for blueline tilefish is not allowed from January 1–April 30 and September 1–December 31 to protect spawning.

Recreational fishing for blueline tilefish in federal waters from the Virginia/North Carolina border through Maine historically has opened May 15 and lasted through November 14, which largely coincides with biologists’ understandings about that population’s spawning season. 

Immature females are rare in scientific sampling, which means some uncertainty remains regarding the proportion of fish in these populations that are sexually mature at specific ages or lengths. Understanding immature female blueline tilefish is critical, because they are essential for accurately estimating sexual maturity and spawning potential — which directly determines sustainable fishing limits.

What did they study?

A multi-institutional research team collaborated with two charter boat operators to target small blueline tilefish (shorter than 15.75 inches in total length) off the coast of Cape Hatteras, as well as in submarine canyons in the vicinity of the Mid-Atlantic continental slope. 

They then microscopically studied the tissue of testes and ovaries to evaluate reproductive development. Researchers also used a microscope to examine otoliths (inner ears of bony fish), which can be “aged” like trees by counting concentric growth rings. This information allowed the team to determine age and length at maturity. 

The researchers combined this new data set with known data sets on age and reproduction from fish sampled from both the South Atlantic and Mid-Atlantic regions to inform a statistical model and infer overall spawning season and sex ratios. 

What did they find?

Blueline tilefish in the South Atlantic are long-lived (to 50+ years), and the population includes a nearly equal proportion of female to male fish overall (1.1 to 1.0). At larger sizes (when the total length is greater than 25.5 inches), males were more abundant than females.

Mid-Atlantic samples consisted mostly of younger fish, ranging from 1 to 5 years old, with females outnumbering males nearly two-fold. As with the South Atlantic fish, the team once again observed more males at larger sizes (in this case over 27.5 inches). 

For both regions, sampling included more females during spawning months (April to October).

The researchers also determined that 50% of female blueline tilefish are mature at roughly 13 inches in total length and at 3 years old. Spawning females were present from March to November in the South Atlantic and May to November in the Mid-Atlantic region. 

The weights of testes and ovaries (indicators of sexual maturity) showed peaks around May and June in the South Atlantic and August and September in the Mid-Atlantic.

There was limited evidence to support hermaphroditism in the fish, which is the ability of an individual to express both male and female reproductive functions. Evidence of female-to-male sex reversal would have indicated an advantage for the species in overcoming the challenges of their populations (i.e., having more females than males).

Anything else?

Targeted sampling with charter boat operators using a smaller hook size resulted in the capture of a higher percentage of smaller fish than with the traditional sampling (16.4% versus 7.4%). Despite targeted sampling, however, the research team only caught nine total immature fish. 

So what?

This study confirmed a protracted spawning season in both regions that extends into November and also showed a delay in the start of spawning season for Mid-Atlantic fish. It highlights improved methods for identifying spawning females as well, in cooperation with the fishing industry, and argues for inclusion of all reproductive phases when analyzing the maturity of individual fish. 

Population modeling for this species is challenging, as the largest, oldest individuals are often males, making them highly vulnerable to overfishing, which can skew sex ratios and reduce reproductive potential. 

The National Oceanic and Atmospheric Administration’s Southeast Cooperative Research Program supported this work under Grant No. NA21NMF4540280.

Reading

Low, J.G., Wyanski, D.M., Klibansky, N., Henley, W., Shertzer, K.W., Cimino, J., Spanik, K.R., Schmidtke, M.A., Wong, C.C., and Jiao, Y. (2026) Maturity, spawning seasonality, and sex ratios of U.S. Atlantic blueline tilefish. Marine and Coastal Fisheries. 18(1). https://doi.org/10.1093/mcfafs/vtaf046

lead image: tilefish (Caulolatilus microps). 
Photo credit: Andrew David, NOAA/NMFS/SEFSC Panama City; Lance Horn, UNCW/NURC – Phantom II ROV operator.

The text from Hook, Line & Science is available to reprint and republish at no cost, but only in its entirety and with this attribution: Hook, Line & Science, courtesy of Scott Baker and Sara Mirabilio, North Carolina Sea Grant.