My previous post was about Lunella torquata data. This post is about Turbo militaris, the second species in my research. For all sites except Mahon Pool, these snails were collected, measured and tagged opportunistically if they were encountered.
At Mahon Pool, after two tagging episodes, it became obvious that juvenile Turbo militaris were highly dominant at this site, so I targeted them to get juvenile growth rates. They are also very numerous on the reefs offshore from this man-made, rock-platform pool compared to my other sites. The pool, being artificial, is not a typical Turbinid habitat, so growth data from this site need to be treated with caution.
The overall recapture data for Mahon Pool in Figure 1 below show the difference between the two species in the pool: the number of snails, (88% of my Turbo militaris recaptures are from this site), the smaller size of Turbo militaris, its higher monthly growth rate, and the non-linearity of juvenile Turbo growth compared to the linearity of Lunella data.
Below is a summary of all my Turbo militaris tagging and recapture data. A summary of all my length frequency measurement data will appear in a later blog post.
TABLE 1. TAGGING AND RECAPTURE DATA SUMMARY FOR Turbo militaris FROM APRIL 2018 TO APRIL 2024 AT 9 SITES IN 5 LOCATIONS.
SITE | Number tagged #1 (multiple taggings in brackets) | Single re-captures and recapture rate #2 Multiple tag re-captures (in brackets) #3 | Minimum, Mean and Maximum sizes at tagging of recaptured snails (L1) | Minimum, mean and maximum sizes of recaptured snails (L2) | Minimum, mean and maximum months at liberty (dT) |
All data | 2,677 | 1090 40.7% | |||
Congwong Bay 1 C1 | 19 | 3, 15.8% | 66, 68.8, 72 | 72.5, 75, 77 |
|
Congwong Bay 2 C2 | 7 | 0, 0% | NIL | NIL | NIL |
Congwong Bay 3 C3 | 22 | 5, 22.7% | 41, 65.1, 81 | 43.5, 68.8, 91 | 0.5, 1.4, 5.1 |
Kurnell 1 | 145 | 20, 13.8% | 49, 76.4, 104 | 56, 87.7, 112 | 2, 6.9, 20.3 |
Kurnell 2 | 125 | 15, 12.0% | 63, 85, 113 | 66, 88.2, 115 | 1.9, 4.7, 9.1 |
Yarra Bay | 130 | 41, 31.5% | 46, 79, 106 | 61, 85, 107 | 3, 5.8, 11.7 |
Little Bay 1 | 229 | 46, 20.1% | 22, 63.2, 96 | 43, 73.4, 100 | 0.8, 6.6, 22.1 |
Little Bay 2 | 66 | 1 1.5% | 67.5 | 74 | 6.5 |
Mahon Pool | 1934 (377) | 959, (135) 49.6% | 13, 36, 92 | 18, 50, 94.5 | 1, 5.4, 24.3 |
Notes: Recapture data only include “usable” recaptures and do not include recaptures where transcription errors resulted in uncertainty about size or date of tagging, where the recaptured snail was dead (an empty shell), or if the average monthly growth increment was above 5mm (an arbitrary figure suggesting some sort of data error). In all, there are 1,104 entries in my recaptured snail list, but 14 are not used in my analyses.
Note #1. This includes all snails tagged including some multiple taggings at Mahon Pool (ie snails recaptured with a tag which were re-tagged with a different coloured tag and returned to the water). Snails tagged twice appear in these numbers of taggings twice, and those tagged three times appear in the list three times. Thus the number of individual snails is much lower than these numbers for Mahon Pool.
Note #2. “single recaptures” means any usable recapture where a length at tagging and a length at recapture and both dates could be determined. They include sequential recaptures of a snail which was re-tagged and returned to the water. A “single recapture” data set is L1,L2,T1,T2. Recapture rate is number of such recapture data sets divided by the number of times that snails were tagged and returned to the water, including multiple taggings.
Note #3. “multiple recaptures” are the number of usable length-at-tagging/length-at-recapture pairs where a single snail was recaptured more than once. This only occurred for Turbo from May 2021 at Mahon Pool where my tag recapture rate was high. I did this to provide a small subset of multiple tag/multiple recapture data. Statistically, including these 135 data points gives these snails more weight in the analysis than snails which were only tagged once and recaptured once. The degree to which any analysis might be confounded will require some partitioned analyses. Initially, for these multiple recaptures, only the lowest L1 and the highest L2 were used – the intermediate lengths and dates were only used for two separate, specific analyses. One was to estimate measurement error (resulting mean = 0.6 -0.7mm – see future post). The recapture rate of repeatedly-tagged T. militaris snails was 50%.
Figure 2 below shows a snail that has been tagged with red rectangles, then upon recapture, re-tagged with orange rectangles, then on a second recapture date, with blue rectangles. The data used for such a recapture are the length and date when red tags were attached, and the length and date at final recapture (lengths for orange and blue tags were generally ignored).
Figure 2. Turbo militaris from Mahon Pool which was tagged and recaptured 3 times and therefore measured 4 times.
Figure 3. Distribution of Turbo militaris recaptures for up to the months at liberty shown.
There are several issues associated with “time at liberty”. Some will be covered in future blog posts about results, but one interesting aspect is leaving small snails out there too long. If double or triple-tagged, it is more likely that anterior tags will be lost. It is also likely that the ventral tags (which generally last longest) will be covered by new nacre as the snail grows in its spiral pattern. Figure 4 below shows the third, ventral red tag almost covered by nacre. This can compromise the estimation of length at tagging on small snails, especially if one or two of the dorsal tags are lost.
My research was always meant to be a simple study with basic research questions:
- What is the growth rate at various sizes?
- Does it vary seasonally?
- Are sites different?
- Does growth vary between years?
- Is growth different for the 2 species?
- Are males and females different?
This summary of recaptures should be viewed in that context.
As to the last question, male and female recaptures are critical. Analysis of early data for Lunella torquata (Kienzle, Broadhurst and Hamer, 2022) showed clearly that males and females grew differently (272 males, 184 females). My previous post summarizing my final Lunella torquata data suggests the difference is confirmed by the larger data set.
For Turbo militaris however, initial analyses suggest there is little difference.
Figure 5 below shows raw data plots for Turbo militaris from 4 locations combined. The regression coefficients (and therefore the growth parameter estimates) are very similar for females (5a) and males (5b). Note however, that these do not include Mahon Pool recaptures.
Figure 6 below shows Ford/Walford plots for male and female Turbo militaris snails at liberty for around one year (see blog post 4 on Growth Rate Issues) for all sites combined, including Mahon Pool to get reasonable numbers.
Unlike the plots in Fig 5 above, these Figure 6 plots do seem to indicate a slight difference between males and females.
Accordingly, as for Lunella torquata, nearly all future analyses of these recapture data addressing the first 5 research questions will depend on the answer to the 6th question about sex differences. A basic summary of the sex data is shown in Table 2 below.
TABLE 2. RECAPTURE DATA SUMMARY FOR MALE AND FEMALE Turbo militaris FROM APRIL 2018 TO APRIL 2024 AT 8 SITES IN 5 LOCATIONS.
SITE | NUMBER OF FEMALES | NUMBER OF MALES | NUMBER WITH UNKNOWN SEX OR JUVENILES OR NOT SEXED #1 (U,J orN) | MEAN LENGTHS AT TAGGING AND RECAPTURE (L1,L2) AND MEAN MONTHS AT LIBERTY (dT) | ||||
FEMALES | MALES | |||||||
All data n=1090 | 188 | 245 | 657 | 47.8 61.0 6.2 | 48.1 60.1 5.9 | |||
Congwong Bay 1 & 3 C1, C3 | 1 | 5 | 2 |
| 67.6 71.5 1.7 | |||
Kurnell 1 & 2 K1, K2 | 13 | 20 | 2 | 80.4 91.2 6.9 | 79.1 85.6 5.2 | |||
Yarra Bay | 10 | 23 | 8 | 77.2 82.5 5.2 | 80.1 86.3 6.1 | |||
Little Bay 1 & 2 LB1, LB2 | 22 | 16 | 9 | 71.3 81.5 7.7 | 59.4 71.8 6.6 | |||
Mahon Pool | 142 | 181 | 636 #2 | 39.1 53.5 6.0 | 39 52.7 6.0 | |||
Note #1 : Initially, non-female and non-male recaptures were recorded as U(nknown). Most of these were snails whose gonads could not be extracted after boiling. Later, J(uveniles) were recorded separately, and more recently, (Note #2), recaptures which were re-tagged and not kept were recorded as N(ot sexed). There were many of these at Mahon Pool.
The mean increment of 2.13mm/month for males was only marginally less than for females (2.24mm/month). Since Figures 4 and 5 are primitive and simplistic analyses, and Mahon Pool returns may not reflect typical growth, future analysis of variance or log-likelihood estimates will be needed to determine whether there is a significant difference in growth between sexes.
References
Gulland J A and S J Holt. 1959. Estimation of growth parameters for data at unequal time intervals. J.Cons.Int.Explor.Mer 25(1):47-49
Kienzle M, M Broadhurst and G Hamer, 2022. Bayesian estimates of turban snail (Lunella torquata) growth off south-eastern Australia. Fisheries Research : 248, 106218.) https://doi.org/10.1016/j.fishres.2021.106218