Dear colleagues, We are pleased to announce our recent publication: Bortolotto G.A., Danilewicz D., Andriolo A. and Zerbini A.N. (2016). Humpback whale *Megaptera novaeangliae* (Cetartiodactyla: Balaenopteridae) group sizes in line transect ship surveys: An evaluation of observer errors. *Zoologia (Curitiba)*, 33(2), e20150133. In this *short communication* we describe a study were we aimed to evaluate the potential errors in group size estimation during line transect ship surveys to estimate abundances of the humpback whale. ABSTRACT. Accurate estimates of group sizes through line transect sampling methods are important to correctly ascertain the abundance of animals that occur in groups. Since the average observed group size is a component of the distance sampling formula, bias in these data leads to biased abundance estimates. This study aimed to evaluate the potential errors in group size estimation during line transect ship surveys to estimate abundances of the humpback whale,* Megaptera novaeangliae* (Borowski 1781). In a research cruise along the Brazilian coast, an experiment to collect group size information was conducted from two different sighting platforms on the same vessel. Group sizes were recorded by primary observers at first sighting ( *PO1*) and, in some cases, after some time (*PO2*). A tracker (*T*) was located on a higher platform to estimate the sizes of groups detected by the primary observers, but tracked one group at a time until it passed abeam. Thus, the dedicated effort to obtain multiple group counts (i.e. higher platform, more time and no responsibility for detecting new groups) was expected to provide more accurate numbers. *PO2* estimates were compared with *PO1* estimates, and *T* estimates were compared with both *PO1* and *PO2*. Additionally, ratios between* T* and both *PO2* (*R1*) and *PO1* (*R2*), and between *PO2 *and *PO1* (R3) were calculated. To investigate a possible improvement in abundance estimates, a correction factor (*CF*) was computed from the ratio of *T* and *PO2* means. Primary observer self-correction (= 1.60, CV% = 70.3) was statistically similar to the correction for the tracker (= 1.62, CV% = 84.1). *CF* resulted in 1 and would not improve abundance estimates. This study supports that observers conducting line transect surveys on large whales have the potential to provide group size information that is as adequate as the correction procedure adopted. The paper is available online here: http://www.scielo.br/pdf/zool/v33n2/1984-4689-zool-33-02-e20150133.pdf With kind regards, *Gui Bortolotto* *|* PhD Student SMRU • CREEM • School of Biology University of St Andrews http://biology.st-andrews.ac.uk/contact/staffProfile.aspx?sunid=gabdo Mobile UK: (+44) 0 7884 398394 Email: gabdo@st-andrews.ac.uk R3 Animal Association (http://en.r3animal.org/)