`dht2` cannot produce estimates of group density/abundance

[erex]

When given data for animals observed in groups, dht2 only produces estimates of individual density and abundance. It does not produce estimates of group density or abundance.

data("ClusterExercise")
minke <- ds(ClusterExercise, key="hr", formula=~size)
summary(minke)
minke.dht2 <- dht2(minke, flatfile = ClusterExercise, strat_formula = ~Region.Label)
print(minke.dht2, report="both")

produces

> print(minke.dht2, report="both")
Abundance and density estimates from distance sampling
Stratification : geographical 
Variance       : R2, n/L 
Multipliers    : none 
Sample fraction : 1 

Summary statistics:
 Region.Label   Area CoveredArea  Effort  n  k    ER se.ER cv.ER
        North 630582    5270.515 1358.38 51 12 0.038 0.014 0.375
        South  84734    1879.511  484.41 39 13 0.081 0.018 0.225
        Total 715316    7150.025 1842.79 90 25 0.049 0.032 0.662

Abundance estimates:
 Region.Label Estimate        se    cv   LCI   UCI     df group_mean  group_se
        North    25303 10561.081 0.417 10856 58976 16.816   2.411765 0.3477394
        South     7064  1973.339 0.279  4027 12393 27.362   2.153846 0.2905123
        Total    32367 10916.685 0.337 16291 64308 19.190   2.282805 0.4531226

Density estimates:
 Region.Label Estimate    se    cv    LCI    UCI     df group_mean  group_se
        North   0.0401 0.017 0.417 0.0172 0.0935 16.816   2.411765 0.3477394
        South   0.0834 0.023 0.279 0.0475 0.1463 27.362   2.153846 0.2905123
        Total   0.0452 0.015 0.337 0.0228 0.0899 19.190   2.282805 0.4531226

Component percentages of variance:
 Region.Label Detection    ER Groups
        North      7.42 80.64  11.93
        South     11.92 64.76  23.31
        Total      2.44 89.50   8.06